View source for 4

{{Short description|Integer number 4}}
{{Hatnote|This article is about the number. For the years, see BC 4 and 4 AD. For other uses, see 4 (disambiguation), IV (disambiguation) and Number 4.}}
{{Infobox number
|number=4
|numeral=[[Quaternary numeral system|quaternary]]
|divisor=1, 2, 4
|roman =IV, iv
|greek prefix=tetra-
|latin prefix=quadri-/quadr-
|lang2=[[Eastern Arabic numerals|Arabic]], Kurdish
|lang2 symbol={{resize|150%|٤}}
|lang3=[[Social:Persian language|Persian]], [[Social:Sindhi language|Sindhi]]
|lang3 symbol={{resize|150%|{{lang|sd|۴}}}}
|lang4=Shahmukhi, Urdu
|lang4 symbol={{resize|150%|{{lang|ur|{{nq|۴}}}}}}
|lang5=Ge'ez
|lang5 symbol={{resize|150%|፬}}
|lang6=[[Social:Bengali language|Bengali]], [[Social:Assamese language|Assamese]]
|lang6 symbol={{resize|150%|৪}}
|lang7=Chinese numeral
|lang7 symbol=四，亖，肆
|lang9=Devanagari
|lang9 symbol={{resize|150%|४}}
|lang10=[[Social:Telugu language|Telugu]]
|lang10 symbol={{resize|150%|౪}}
|lang11=[[Social:Malayalam|Malayalam]]
|lang11 symbol={{resize|150%|൪}}
|lang12=[[Social:Tamil language|Tamil]]
|lang12 symbol={{resize|150%|௪}}
|lang13=Hebrew
|lang13 symbol={{resize|150%|ד}}
|lang14=[[Khmer numerals|Khmer]]
|lang14 symbol={{resize|150%|៤}}
|lang15=[[Thai numerals|Thai]]
|lang15 symbol={{resize|150%|๔}}
|lang16=Kannada
|lang16 symbol={{resize|150%|೪}}
|lang17=[[Social:Burmese language|Burmese]]
|lang17 symbol={{resize|150%|၄}}
}}

'''4''' ('''four''') is a [[Number|number]], [[Philosophy:Numeral (linguistics)|numeral]] and [[Numerical digit|digit]]. It is the [[Natural number|natural number]] following [[3|3]] and preceding [[5|5]]. It is a [[Square number|square number]], the smallest [[Semiprime|semiprime]] and [[Composite number|composite number]], and is [[Unsolved:Tetraphobia|considered unlucky]] in many East Asian cultures.

==Evolution of the Hindu-Arabic digit==
[[File:Evolution4glyph.png|x50px|left]]
[[File:Vier.jpg|thumb|120px|left|Two modern handwritten fours]]
Brahmic numerals represented 1, 2, and 3 with as many lines. 4 was simplified by joining its four lines into a cross that looks like the modern plus sign. The [[History:Shunga Empire|Shunga]] would add a horizontal line on top of the digit, and the Kshatrapa and Pallava evolved the digit to a point where the speed of writing was a secondary concern. The Arabs' 4 still had the early concept of the cross, but for the sake of efficiency, was made in one stroke by connecting the "western" end to the "northern" end; the "eastern" end was finished off with a curve. The Europeans dropped the finishing curve and gradually made the digit less cursive, ending up with a digit very close to the original Brahmin cross.<ref>Georges Ifrah, ''The Universal History of Numbers: From Prehistory to the Invention of the Computer'' transl. David Bellos et al. London: The Harvill Press (1998): 394, Fig. 24.64</ref>

While the shape of the character for the digit 4 has an ascender in most modern [[Typeface|typeface]]s, in typefaces with [[Text figures|text figures]] the glyph usually has a descender, as, for example, in [[File:TextFigs148.svg|40px]].

On the [[Engineering:Seven-segment display|seven-segment display]]s of pocket calculators and digital watches, as well as certain [[Optical character recognition|optical character recognition]] fonts, 4 is seen with an open top: [[File:Seven-segment 4.svg|x25px]].<ref>{{Cite web|date=2019-04-22|title=Seven Segment Displays (7-Segment) {{!}} Pinout, Types and Applications|url=https://www.electronicshub.org/seven-segment-displays/|access-date=2020-07-28|website=Electronics Hub|language=en-US|archive-date=28 July 2020|archive-url=https://web.archive.org/web/20200728014108/https://www.electronicshub.org/seven-segment-displays/|url-status=live}}</ref>

[[Engineering:Television station|Television station]]s that operate on channel 4 have occasionally made use of another variation of the "open 4", with the open portion being on the side, rather than the top. This version resembles the Canadian Aboriginal syllabics letter ᔦ. The [[Magnetic ink character recognition|magnetic ink character recognition]] "CMC-7" font also uses this variety of "4".<ref>{{Cite news|date=2017-02-02|title=Battle of the MICR Fonts: Which Is Better, E13B or CMC7? - Digital Check|language=en-US|work=Digital Check|url=https://www.digitalcheck.com/battle-micr-fonts-better-e13b-cmc7/|access-date=2020-07-28|archive-date=3 August 2020|archive-url=https://web.archive.org/web/20200803161254/https://www.digitalcheck.com/battle-micr-fonts-better-e13b-cmc7/|url-status=live}}</ref>

==Mathematics ==
Four is the smallest [[Composite number|composite number]], its proper [[Divisor|divisor]]s being {{num|1}} and {{num|2}}.<ref>{{Cite book| last=Fiore | first=Gregory | url=https://books.google.com/books?id=moE01lKtPJcC&q=Four+is+the+smallest+composite+number,|title=Basic mathematics for college students: concepts and applications|date=1993-08-01|publisher=HarperCollins College | isbn=978-0-06-042046-8 |pages=162|language=en|quote=The smallest composite number is 4.}}</ref> Four is the sum and product of [[Two|two]] with itself: <math>2+2=4=2\times 2</math>, the only non-zero number <math>b</math> such that <math>a+a=b=a\times a</math>, which also makes four the smallest and only even squared [[Prime number|prime number]] <math>2^{2}</math> and hence the first squared prime of the form <math>p^2</math>, where <math>p</math> is a prime. Four, as the first composite number, has a prime aliquot sum of 3; and as such it is part of the first [[Aliquot sequence|aliquot sequence]] with a single composite member, expressly (4, 3, [[1|1]], [[0|0]]).

* In [[Knuth's up-arrow notation]], <math>2+2=2\times2=2^{2}=2\uparrow\uparrow 2=2\uparrow\uparrow\uparrow2=\;...\; = 4</math>, and so forth, for any number of up arrows.<ref>{{Cite book|last=Hodges|first=Andrew| url=https://books.google.com/books?id=HOcpgfiDu40C&q=2+%E2%86%91%E2%86%91+2&pg=PA249|title=One to Nine: The Inner Life of Numbers| date=2008-05-17|publisher=W. W. Norton & Company|isbn=978-0-393-06863-4|pages=249|language=en|quote=2 ↑↑ ...  ↑↑ 2 is always 4}}</ref> By consequence, four is the only square one more than a prime number, specifically [[3|three]]. 
* The sum of the first four prime numbers [[2|two]] + [[3|three]] + [[5|five]] + [[7|seven]] is the only sum of four consecutive prime numbers that yields an [[Parity (mathematics)|odd]] prime number, [[17 (number)|seventeen]], which is the fourth [[Super-prime|super-prime]]. Four lies between the first proper pair of twin primes, [[3|three]] and [[5|five]], which are the first two Fermat primes, like [[17 (number)|seventeen]], which is the third. On the other hand, the [[Square (algebra)|square]] of four (<math>4^2</math>), equivalently the [[Fourth power|fourth power]] of two (<math>2^4</math>), is [[16 (number)|sixteen]]; the only number that has <math>a^{b}=b^{a}</math> as a form of [[Factorization|factorization]].

Holistically, there are four elementary arithmetic [[Operation (mathematics)|operations]] in mathematics: [[Addition|addition]] ('''+'''), [[Subtraction|subtraction]] ('''−'''), [[Multiplication|multiplication]] ('''×'''), and [[Division (mathematics)|division]] ('''÷'''); and four basic number systems, the [[Real number|real number]]s <math>\mathbb{R}</math>, [[Rational number|rational number]]s <math>\mathbb{Q}</math>, [[Integer|integer]]s <math>\mathbb{Z}</math>, and [[Natural number|natural number]]s <math>\mathbb{N}</math>.

Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e. <math>4x=y^{2}-z^{2}</math>. A number is a multiple of 4 if its last two digits are a multiple of 4 (for example, 1092 is a multiple of 4 because {{nowrap|1=92 = 4 × 23}}).<ref>{{Cite book|author=Kaplan Test Prep| url=https://books.google.com/books?id=D_rLDQAAQBAJ&q=last+two+digits+are+a+multiple+of+4&pg=PA289|title=SAT Subject Test Mathematics Level 1|date=2017-01-03|publisher=Simon and Schuster|isbn=978-1-5062-0922-7|pages=289|language=en|quote=An integer is divisible by 4 if the last two digits form a multiple of 4.}}</ref>

[[Lagrange's four-square theorem]] states that every positive integer can be written as the sum of at most four [[Square number|square number]]s.<ref>{{Citation|last=Spencer|first=Joel|title=Four Squares with Few Squares|year=1996|work=Number Theory: New York Seminar 1991–1995|pages=295–297|editor-last=Chudnovsky|editor-first=David V.|place=New York, NY|publisher=Springer US| language=en|doi=10.1007/978-1-4612-2418-1_22|isbn=978-1-4612-2418-1|editor2-last=Chudnovsky|editor2-first=Gregory V.|editor3-last=Nathanson|editor3-first=Melvyn B.}}</ref> Three are not always sufficient; {{num|7}} for instance cannot be written as the sum of three squares.<ref>{{Cite book|last=Peterson|first=Ivars|url=https://books.google.com/books?id=4gWSAraVhtAC&q=7+for+instance+cannot+be+written+as+the+sum+of+three+squares.&pg=PA95|title=Mathematical Treks: From Surreal Numbers to Magic Circles|date=2002|publisher=MAA|isbn=978-0-88385-537-9|pages=95|language=en|quote=7 is an example of an integer that can't be written as the sum of three squares.}}</ref>

There are four [[Harshad number|all-Harshad number]]s: [[1|1]], [[2|2]], ''4'', and [[6|6]]. [[12 (number)|12]], which is divisible by four thrice over, is a Harshad number in all bases except [[Octal|octal]].

A four-sided plane figure is a [[Quadrilateral|quadrilateral]] or quadrangle, sometimes also called a ''tetragon''. It can be further classified as a [[Rectangle|rectangle]] or ''oblong'', [[Engineering:Kite|kite]], [[Rhombus|rhombus]], and [[Square|square]].

Four is the highest degree general polynomial equation for which there is a [[Solution in radicals|solution in radicals]].<ref>{{Cite book| last=Bajnok|first=Béla|url=https://books.google.com/books?id=cNFzKnvxXoAC&q=Abel%E2%80%93Ruffini+theorem&pg=PT78|title=An Invitation to Abstract Mathematics|date=2013-05-13|publisher=Springer Science & Business Media| isbn=978-1-4614-6636-9| language=en|quote=There is no algebraic formula for the roots of the general polynomial of degrees 5 or higher.}}</ref>

The four-color theorem states that a [[Planar graph|planar graph]] (or, equivalently, a flat [[Earth:Map|map]] of two-dimensional regions such as countries) can be colored using four colors, so that adjacent vertices (or regions) are always different colors.<ref>{{cite book |first=Bryan |last=Bunch |title=The Kingdom of Infinite Number |location=New York |publisher=W. H. Freeman & Company |year=2000 |page=48}}</ref> Three colors are not, in general, sufficient to guarantee this.<ref>{{Cite book|last=Ben-Menahem|first=Ari|url=https://books.google.com/books?id=9tUrarQYhKMC&q=three+colors+map+not+enough&pg=PA2147|title=Historical Encyclopedia of Natural and Mathematical Sciences|date=2009-03-06|publisher=Springer Science & Business Media| isbn=978-3-540-68831-0|pages=2147|language=en|quote=(i.e. That there are maps for which three colors are not sufficient)}}</ref> The largest planar [[Complete graph|complete graph]] has four vertices.<ref>{{Cite book|last=Molitierno|first=Jason J.| url=https://books.google.com/books?id=2kvNBQAAQBAJ&q=largest+planar+complete+graph+has+four+vertices&pg=PA197| title=Applications of Combinatorial Matrix Theory to Laplacian Matrices of Graphs|date=2016-04-19|publisher=CRC Press| isbn=978-1-4398-6339-8|pages=197|language=en|quote=... The complete graph on the largest number of vertices that is planar is K4 and that a(K4) equals 4.}}</ref>

A solid figure with four faces as well as four vertices is a [[Tetrahedron|tetrahedron]], which is the smallest possible number of faces and vertices a [[Polyhedron|polyhedron]] can have.<ref>{{Cite web|last=Weisstein|first=Eric W.|title=Tetrahedron|url=https://mathworld.wolfram.com/Tetrahedron.html|access-date=2020-07-28|website=mathworld.wolfram.com|language=en|archive-date=20 August 2020|archive-url=https://web.archive.org/web/20200820085424/https://mathworld.wolfram.com/Tetrahedron.html|url-status=live}}</ref><ref>{{Cite book|last1=Grossnickle|first1=Foster Earl|url=https://books.google.com/books?id=Q2474oSAsc4C&q=4+is+the+smallest+possible+number+of+faces+(as+well+as+vertices)+of+a+polyhedron.|title=Discovering Meanings in Elementary School Mathematics|last2=Reckzeh|first2=John|date=1968|publisher=Holt, Rinehart and Winston|pages=337|isbn=9780030676451|language=en|quote=...the smallest possible number of faces that a polyhedron may have is four}}</ref> The regular tetrahedron, also called a 3-[[Simplex|simplex]], is the simplest [[Platonic solid]].<ref>{{Cite book|last1=Grossnickle|first1=Foster Earl|url=https://books.google.com/books?id=Q2474oSAsc4C&q=4+is+the+smallest+possible+number+of+faces+(as+well+as+vertices)+of+a+polyhedron.|title=Discovering Meanings in Elementary School Mathematics|last2=Reckzeh|first2=John|date=1968|publisher=Holt, Rinehart and Winston|pages=337|isbn=9780030676451|language=en|quote=...face of the platonic solid. The simplest of these shapes is the tetrahedron...}}</ref> It has four regular triangles as faces that are themselves at dual positions with the vertices of another tetrahedron.<ref>{{Cite book|last1=Hilbert|first1=David|url=https://books.google.com/books?id=7WY5AAAAQBAJ&q=self-dual+regular+polyhedron&pg=PA143|title=Geometry and the Imagination|last2=Cohn-Vossen |first2=Stephan |date=1999|publisher=American Mathematical Soc.|isbn=978-0-8218-1998-2|pages=143|language=en|quote=...the tetrahedron plays an anomalous role in that it is self-dual, whereas the four remaining polyhedra are mutually dual in pairs...}}</ref> Tetrahedra can be inscribed inside all other four Platonic solids, and [[Tessellation|tessellate space]] alongside the regular octahedron in the alternated cubic honeycomb.

The third dimension holds a total of four [[Coxeter group]]s that generate convex [[Uniform polyhedron|uniform polyhedra]]: the tetrahedral group, the octahedral group, the icosahedral group, and a [[Dihedral group|dihedral group]] (of orders 24, 48, 120, and 4<math>n</math>, respectively). There are also four general Coxeter groups of generalized uniform prisms, where two are [[Hosohedron|hosoderal]] and dihedral groups that form spherical tilings, with another two general [[Prism (geometry)|prismatic]] and [[Antiprism|antiprism]]atic groups that represent [[Truncation|truncated]] hosohedra (or simply, [[Prism (geometry)|prisms]]) and snub antiprisms, respectively.

[[Four-dimensional space]] is the highest-dimensional space featuring more than three [[Regular polytope|regular]] [[Convex polytope|convex]] figures: 
*Two-dimensional: infinitely many [[Regular polygon|regular polygon]]s.
*Three-dimensional: five [[Regular polyhedron|regular polyhedra]]; the five [[Platonic solid]]s which are the [[Tetrahedron|tetrahedron]], [[Cube|cube]], [[Octahedron|octahedron]], [[Dodecahedron|dodecahedron]], and [[Icosahedron|icosahedron]]. 
*Four-dimensional: six regular polychora; the [[5-cell|5-cell]], 8-cell or [[Tesseract|tesseract]], [[16-cell|16-cell]], [[24-cell|24-cell]], [[120-cell|120-cell]], and [[600-cell|600-cell]]. The [[24-cell|24-cell]], made of regular octahedra, has no analogue in any other dimension; it is self-dual, with its [[24-cell honeycomb|24-cell honeycomb]] dual to the [[16-cell honeycomb|16-cell honeycomb]]. 
*Five-dimensional and every higher dimension: three regular convex <math>n</math>-[[Polytope|polytope]]s, all within the infinite family of regular <math>n</math>-[[Simplex|simplex]]es, <math>n</math>-[[Hypercube|hypercube]]s, and <math>n</math>-[[Cross-polytope|orthoplexes]].

The fourth dimension is also the highest dimension where regular self-intersecting figures exist:
*Two-dimensional: infinitely many regular [[Star polygon|star polygon]]s.
*Three-dimensional: ''four'' regular [[Star polyhedron|star polyhedra]], the [[Regular polytope|regular]] Kepler-Poinsot star polyhedra. 
*Four-dimensional: ten regular [[Star polyhedron#Star polytopes|star polychora]], the [[Regular 4-polytope#Regular star (Schläfli–Hess) 4-polytopes|Schläfli–Hess star polychora]]. They contain cells of Kepler-Poinsot polyhedra alongside regular tetrahedra, icosahedra and [[Regular dodecahedron|dodecahedra]].
*Five-dimensional and every higher dimension: zero regular [[Star polyhedron#Star polytopes|star-polytopes]]; uniform star polytopes in dimensions <math>n</math> > <math>4</math> are the most symmetric, which mainly originate from [[Stellation|stellation]]s of regular <math>n</math>-polytopes.

Altogether, [[16 (number)|sixteen]] (or 16 = '''4<sup>2</sup>''') regular convex and star polychora are generated from symmetries of ''four'' ('''4''') [[Coxeter group|Coxeter]] [[Weyl group]]s and [[Point groups in four dimensions|point groups]] in the fourth dimension: the [[A4 polytope|<math>\mathrm A {4}</math>]] simplex, [[B4 polytope|<math>\mathrm B {4}</math>]] hypercube, [[List of F4 polytopes|<math>\mathrm F {4}</math>]] icositetrachoric, and [[H4 polytope|<math>\mathrm H {4}</math>]] hexacosichoric groups; with the [[D4 polytope|<math>\mathrm D {4}</math>]] [[Demihypercube|demihypercube]] group generating two alternative constructions. There are also [[64 (number)|sixty-four]] (or 64 = '''4<sup>3</sup>''') four-dimensional [[Bravais lattice]]s, alongside sixty-four uniform polychora in the fourth dimension based on the same <math>\mathrm A_{4}</math>, <math>\mathrm B_{4}</math>, <math>\mathrm F_{4}</math> and <math>\mathrm H_{4}</math> [[Point groups in four dimensions|Coxeter groups]], and extending to prismatic groups of [[Uniform polyhedron|uniform polyhedra]], including one special [[Wythoff construction|non-Wythoffian form]], the [[Grand antiprism|grand antiprism]]. Two infinite families of [[Duoprism|duoprism]]s and [[Uniform antiprismatic prism|antiprismatic prisms]] exist in the fourth dimension.

There are only four [[Polytope|polytope]]s with radial equilateral symmetry: the [[Hexagon|hexagon]], the [[Cuboctahedron|cuboctahedron]], the tesseract, and the 24-cell.

Four-dimensional differential manifolds have some unique properties. There is only one differential structure on <math>\mathbb{R}^n</math> except when <math>n</math> = <math>4</math>, in which case there are uncountably many.

The smallest non-[[Cyclic group|cyclic group]] has four elements; it is the [[Klein four-group]].<ref>{{Cite book|first=Jeremy|last=Horne|url=https://books.google.com/books?id=ZfYoDwAAQBAJ&pg=PA299|title=Philosophical Perceptions on Logic and Order|date=2017-05-19|publisher=IGI Global|isbn=978-1-5225-2444-1|pages=299|language=en|quote=The Klein four-group is the smallest noncyclic group,...|access-date=31 October 2022|archive-date=31 October 2022|archive-url=https://web.archive.org/web/20221031005437/https://books.google.com/books?id=ZfYoDwAAQBAJ&pg=PA299|url-status=live}}</ref> ''A{{sub|n}}'' [[Alternating group|alternating group]]s are not [[Simple group|simple]] for values <math>n</math> ≤ <math>4</math>.

There are four [[Hopf fibration]]s of [[Hypersphere|hypersphere]]s:

<math display=block>
\begin{align}
S^0 & \hookrightarrow S^1 \to S^1, \\
S^1 & \hookrightarrow S^3 \to S^2, \\
S^3 & \hookrightarrow S^7 \to S^4, \\
S^7 & \hookrightarrow S^{15}\to S^8. \\
\end{align}</math>

They are defined as locally trivial [[Fibration|fibration]]s that map <math>f : S^{2n-1} \rightarrow S^{n}</math> for values of <math>n=2,4,8</math> (aside from the trivial fibration mapping between two [[Point (geometry)|points]] and a [[Circle|circle]]).<ref>{{Cite book |last=Shokurov |first= A.V. |editor=Michiel Hazewinkel |chapter=Hopf fibration |title=Encyclopedia of Mathematics |publisher=European Mathematical Society |location=Helsinki |chapter-url=https://encyclopediaofmath.org/wiki/Hopf_fibration |year=2002 |isbn=1402006098 |oclc=1013220521 |access-date=2023-04-30 |archive-date=1 May 2023 |archive-url=https://web.archive.org/web/20230501005558/https://encyclopediaofmath.org/wiki/Hopf_fibration |url-status=live }}</ref>

Further extensions of the real numbers under [[Hurwitz's theorem (composition algebras)|Hurwitz's theorem]] states that there are four [[Normed algebra|normed division algebras]]: the real numbers <math>\mathbb{R}</math>, the complex numbers <math>\mathbb C</math>, the [[Quaternion|quaternion]]s <math>\mathbb H</math>, and the [[Octonion|octonion]]s <math>\mathbb O</math>. Under [[Cayley–Dickson construction]]s, the [[Sedenion|sedenion]]s <math>\mathbb S</math> constitute a further fourth extension over <math>\mathbb{R}</math>. The real numbers are [[Ordered field|ordered]], commutative and associative [[Algebraic structure|algebras]], as well as [[Alternative algebra|alternative algebra]]s with power-associativity. The complex numbers <math>\mathbb C</math> share all four multiplicative algebraic properties of the reals <math>\mathbb{R}</math>, without being ordered. The quaternions loose a further commutative algebraic property, while holding associative, alternative, and power-associative properties. The octonions are alternative and power-associative, while the sedenions are only power-associative. The sedenions and all further ''extensions'' of these four normed division algebras are solely power-associative with non-trivial [[Zero divisor|zero divisor]]s, which makes them [[Division algebra|non-division algebras]]. <math>\mathbb{R}</math> has a [[Vector space|vector space]] of [[Physics:Dimension|dimension]] 1, while <math>\mathbb C</math>, <math>\mathbb H</math>, <math>\mathbb O</math> and <math>\mathbb S</math> work in [[Algebraic number field|algebraic number field]]s of dimensions 2, 4, 8, and 16, respectively.

==List of basic calculations==
{|class="wikitable" style="text-align: center; background: white"
|-
!width="105px"|[[Multiplication]]
!1
!2
!3
!4
!5
!6
!7
!8
!9
!10
!width="5px"|
!11
!12
!13
!14
!15
!16
!17
!18
!19
!20
!width="5px"|
!21
!22
!23
!24
!25
!width="5px"|
!50
!100
!1000
|-
|'''4 × ''x'''''
|'''4'''
|{{num|8}}
|{{num|12}}
|{{num|16}}
|{{num|20}}
|{{num|24}}
|{{num|28}}
|{{num|32}}
|{{num|36}}
|{{num|40}}
!
|{{num|44}}
|{{num|48}}
|{{num|52}}
|{{num|56}}
|{{num|60}}
|{{num|64}}
|{{num|68}}
|{{num|72}}
|{{num|76}}
|{{num|80}}
!
|{{num|84}}
|{{num|88}}
|{{num|92}}
|{{num|96}}
|{{num|100}}
!
|{{num|200}}
|{{num|400}}
|{{num|4000}}
|}

{|class="wikitable" style="text-align: center; background: white"
|-
!width="105px"|[[Division (mathematics)|Division]]
!1
!2
!3
!4
!5
!6
!7
!8
!9
!10
!width="5px"|
!11
!12
!13
!14
!15
!16
|-
|'''4 ÷ ''x'''''
|'''4'''
|2
|1.{{overline|3}}
|1
|0.8
|0.{{overline|6}}
|0.{{overline|571428}}
|0.5
|0.{{overline|4}}
|0.4
!
|0.{{overline|36}}
|0.{{overline|3}}
|0.{{overline|307692}}
|0.{{overline|285714}}
|0.2{{overline|6}}
|0.25
|-
|'''''x'' ÷ 4'''
|0.25
|0.5
|0.75
|1
|1.25
|1.5
|1.75
|2
|2.25
|2.5
!
|2.75
|3
|3.25
|3.5
|3.75
|4
|}

{|class="wikitable" style="text-align: center; background: white"
|-
!width="105px"|[[Exponentiation]]
!1
!2
!3
!4
!5
!6
!7
!8
!9
!10
!width="5px"|
!11
!12
!13
!14
!15
!16
|-
|'''4{{sup|''x''}}'''
|4
|16
|64
|256
|1024
|4096
|16384
|65536
|262144
|1048576
!
|4194304
|16777216
|67108864
|268435456
|1073741824
|4294967296
|-
|'''''x''{{sup|4}}'''
|1
|16
|81
|256
|625
|1296
|2401
|4096
|6561
|10000
!
|14641
|20736
|28561
|38416
|50625
|65536
|}

==In religion==
===Buddhism===
*[[Philosophy:Four Noble Truths|Four Noble Truths]] – [[Philosophy:Dukkha|Dukkha]], [[Philosophy:Pratītyasamutpāda|Samudaya]], [[Philosophy:Nirvana|Nirodha]], [[Philosophy:Noble Eightfold Path|Magga]]<ref name=":1">{{Cite book|last=Chwalkowski|first=Farrin|url=https://books.google.com/books?id=W-22DQAAQBAJ&q=Four+main+pilgrimage+sites+Lumbini,+Bodh+Gaya,+Sarnath,+and+Kusinara&pg=PA22|title=Symbols in Arts, Religion and Culture: The Soul of Nature|date=2016-12-14|publisher=Cambridge Scholars Publishing|isbn=978-1-4438-5728-4|pages=22|language=en|quote=The four main pilgrimages sites are: Lumbini, Bodh Gaya, Sarnath and Kusinara....four Noble Truths of Buddhism}}</ref>
*Four sights – observations which affected Prince Siddhartha deeply and made him realize the sufferings of all beings, and compelled him to begin his spiritual journey—an [[Biology:Old age|old]] man, a sick man, a [[Biology:Death|dead]] man, and an ascetic<ref>{{Cite book|last=Van Voorst|first=Robert|url=https://books.google.com/books?id=QvNWxEEaf50C&q=Four+sights+%E2%80%93+observations+which+affected+Prince+Siddhartha&pg=PT123|title=RELG: World|date=2012-01-01|publisher=Cengage Learning|isbn=978-1-111-72620-1|pages=108|language=en|quote=He first observed the suffering of the world in the Four Passing Sites}}</ref>
*[[Philosophy:Mahābhūta|Four Great Elements]] – [[Unsolved:Earth (classical element)|earth]], [[Chemistry:Water (classical element)|water]], [[Unsolved:Fire (classical element)|fire]], and [[Physics:Air (classical element)|wind]]<ref>{{Cite book|last1=Yun|first1=Hsing|url=https://books.google.com/books?id=wcR1nrqxAEgC&q=Four+Great+Elements+%E2%80%93+earth,+water,+fire,+and+wind&pg=PA27|title=The Great Realizations: A Commentary on the Eight Realizations of a Bodhisattva Sutra|last2=Xingyun|date=2010|publisher=Buddha's Light Publishing|isbn=978-1-932293-44-9|pages=27|language=en|quote=The four great elements, earth, water, fire and wind...}}</ref>
*[[Religion:Four Heavenly Kings|Four Heavenly Kings]]<ref>{{Cite book|last=Chaudhuri|first=Saroj Kumar|url=https://books.google.com/books?id=vhAZEZD7weoC&q=Four+Heavenly+Kings|title=Hindu Gods and Goddesses in Japan|date=2003|publisher=Vedams eBooks (P) Ltd|isbn=978-81-7936-009-5|pages=20|language=en|quote=The Buddhists adopted him as one of the four Devarajas or Heavenly Kings}}</ref>
*[[Philosophy:Satipatthana|Four Foundations of Mindfulness]] – contemplation of the body, contemplation of feelings, contemplation of mind, contemplation of mental objects<ref name=":1" />
*[[Philosophy:Four Right Exertions|Four Right Exertions]]<ref>{{Cite book|last=Bronkhorst|first=Johannes|url=https://books.google.com/books?id=Mhuabeq5-cAC&q=Four+Right+Exertions&pg=PA66|title=Buddhist Teaching in India|date=2009-12-22|publisher=Simon and Schuster|isbn=978-0-86171-566-4|pages=66|language=en|quote=The four right exertions are...}}</ref>
*[[Philosophy:Iddhipada|Four Bases of Power]]<ref>{{Cite book|last=Mistry|first=Freny|url=https://books.google.com/books?id=7O_7D8pNTjsC&q=Four+Bases+of+Power+Buddhism&pg=PA69|title=Nietzsche and Buddhism: Prolegomenon to a Comparative Study|date=2011-05-02|publisher=Walter de Gruyter|isbn=978-3-11-083724-7|pages=69|language=en|quote=these four bases of psychic power}}</ref>
*Four jhānas<ref>{{Cite book|last=Arbel|first=Keren|url=https://books.google.com/books?id=UjolDwAAQBAJ&q=Four+jh%C4%81nas|title=Early Buddhist Meditation: The Four Jhanas as the Actualization of Insight|date=2017-03-16|publisher=Taylor & Francis|isbn=978-1-317-38399-4|pages=1|language=en|quote=This book is about the four jhanas}}</ref>
*[[Philosophy:Arūpajhāna|Four arūpajhānas]]<ref>{{Cite book|last=Jayatilleke|first=K. N.|url=https://books.google.com/books?id=FpJTAQAAQBAJ&q=Four+ar%C5%ABpajh%C4%81nas&pg=PT107|title=Early Buddhist Theory of Knowledge|date=2013-10-16|publisher=Routledge|isbn=978-1-134-54294-9|language=en|quote=...the states of the four arupajhanas.}}</ref>
*[[Philosophy:Brahmavihara|Four Divine Abidings]] – [[Philosophy:Mettā|loving-kindness]], [[Philosophy:Karuṇā|compassion]], [[Philosophy:Mudita|sympathetic joy]], and [[Philosophy:Upekkha|equanimity]]<ref>{{Cite book|last=van Gorkom|first=Nina|url=https://books.google.com/books?id=uekVAwAAQBAJ&q=Four+Divine+Abidings+%E2%80%93+loving-kindness,+compassion,+sympathetic+joy,+and+equanimity&pg=PA171|title=The Perfections Leading to Enlightenment|publisher=Рипол Классик|isbn=978-5-88139-786-9|pages=171|language=en|quote=There are four of them: loving-kindness, metta, compassion, karuna, sympathetic joy, mudita and equanimity, upekkha.}}</ref>
*[[Philosophy:Four stages of enlightenment|Four stages of enlightenment]] – [[Philosophy:Sotāpanna|stream-enterer]], [[Philosophy:Sakadagami|once-returner]], non-returner, and arahant<ref>{{Cite book|last1=Rinpoche|first1=Khenchen Konchog Gyaltshen|url=https://books.google.com/books?id=xaRQAgAAQBAJ&q=Four+stages+of+enlightenment+%E2%80%93+stream-enterer,+once-returner,+non-returner,+and+arahant&pg=PT82|title=Opening the Treasure of the Profound: Teachings on the Songs of Jigten Sumgon and Milarepa|last2=Milarepa|last3=Sumgon|first3=Jigten|date=2013-10-08|publisher=Shambhala Publications|isbn=978-0-8348-2896-4|language=en|quote=...four types of shravaka (stream enterer, oncereturner, nonreturner, and arhat)}}</ref>
*Four main pilgrimage sites – Lumbini, Bodh Gaya, Sarnath, and Kusinara<ref name=":1" />

===Judeo-Christian symbolism===
*The Tetragrammaton is the four-letter name of [[Religion:God|God]].<ref>{{Cite book|last1=Fahlbusch|first1=Erwin|url=https://books.google.com/books?id=lZUBZlth2qgC&pg=PA823|title=The Encyclodedia of Christianity, Vol. 5|last2=Bromiley|first2=Geoffrey William|last3=Lochman|first3=Jan Milic|last4=Mbiti|first4=John|last5=Pelikan|first5=Jaroslav|date=2008-02-14|publisher=Wm. B. Eerdmans Publishing|isbn=978-0-8028-2417-2|pages=823|language=en}}</ref>
*Ezekiel has a vision of four [[Religion:Living creatures (Bible)|living creatures]]: a man, a lion, an ox, and an eagle.<ref>{{Cite book|last1=Stevenson|first1=Kenneth|url=https://books.google.com/books?id=Ix_0DAAAQBAJ&q=Ezekiel+has+a+vision+of+four+living+creatures:+a+man,+a+lion,+an+ox,+and+an+eagle.&pg=PR45|title=Ezekiel, Daniel|last2=Glerup|first2=Michael|date=2014-03-19|publisher=InterVarsity Press|isbn=978-0-8308-9738-4|pages=xlv|language=en|quote=We have already mentioned the four living creatures—the man, the lion, the ox and the eagle}}</ref>
*The four Matriarchs (foremothers) of [[Religion:Judaism|Judaism]] are Sarah, Rebekah, Leah, and Rachel.<ref>{{Cite book|last1=Butnick|first1=Stephanie|url=https://books.google.com/books?id=MlSXDwAAQBAJ&q=Matriarchs+(foremothers)+of+Judaism+are+Sarah,+Rebekah,+Leah,+and+Rachel.&pg=PT176|title=The Newish Jewish Encyclopedia: From Abraham to Zabar's and Everything in Between|last2=Leibovitz|first2=Liel|last3=Oppenheimer|first3=Mark|date=2019-10-01|publisher=Artisan Books|isbn=978-1-57965-893-9|language=en|quote=...be like Sarah, Rachel, Rebecca, and Leah, the foremothers of Judaism}}</ref>
*The Four Species (lulav, [[Biology:Hadass|hadass]], aravah and [[Biology:Etrog|etrog]]) are taken as one of the mitzvot on the Jewish holiday of Sukkot. ([[Religion:Judaism|Judaism]])<ref>{{Cite book|last=Kaplan|first=Aryeh|url=https://books.google.com/books?id=NcgRAQAAIAAJ&q=Four+Species+(lulav,+hadass,+aravah+and+etrog)+are+taken+as+one+of+the+mitzvot|title=Innerspace: Introduction to Kabbalah, Meditation and Prophecy|date=1990|publisher=Moznaim|pages=109|isbn=9780940118560|language=en|quote=...as well as to the palm ( lulav ), myrtle ( hadas ), willow ( aravah ) and citron ( etrog ), the four species of plants}}</ref>
*The Four Cups of Wine to drink on the Jewish holiday of Passover. ([[Religion:Judaism|Judaism]])<ref name=":0">{{Cite book|last=Dennis|first=Geoffrey W.|url=https://books.google.com/books?id=Ojl_wsv0x3QC&q=Four+Sons+to+be+dealt+with+on+the+Jewish+holiday+of+Passover&pg=PT204|title=The Encyclopedia of Jewish Myth, Magic and Mysticism|date=2007|publisher=Llewellyn Worldwide|isbn=978-0-7387-0905-5|pages=188|language=en|quote=The Passover Seder is particularly structured around fours: the Four Questions, the Four Sons, and four cups of wine.}}</ref>
*The Four Questions to be asked on the Jewish holiday of Passover. ([[Religion:Judaism|Judaism]])<ref name=":0" />
*The Four Sons to be dealt with on the Jewish holiday of Passover. ([[Religion:Judaism|Judaism]])<ref name=":0" />
*The Four Expressions of Redemption to be said on the Jewish holiday of Passover. ([[Religion:Judaism|Judaism]])<ref>{{Cite web|title=Four Expressions of Redemption to be said on the Jewish holiday of Passover - Google Search|url=https://www.google.com/search?safe=strict&tbm=bks&q=Four+Expressions+of+Redemption+to+be+said+on+the+Jewish+holiday+of+Passover|access-date=2020-07-29|pages=46|quote=There are four expressions of redemption in the Torah|archive-date=25 October 2022|archive-url=https://web.archive.org/web/20221025195338/https://www.google.com/search?safe=strict&tbm=bks&q=Four+Expressions+of+Redemption+to+be+said+on+the+Jewish+holiday+of+Passover|url-status=live}}</ref>
*The four [[Religion:Gospel|Gospel]]s: Matthew, Mark, Luke, and John. ([[Religion:Christianity|Christianity]])<ref>{{Cite book|first=Charles|last=Templeton|url=https://books.google.com/books?id=KywmAQAAMAAJ&q=four+Gospels+Matthew|title=Jesus: the four Gospels, Matthew, Mark, Luke, and John, combined in one narrative and rendered in modern English|date=1973|publisher=Simon and Schuster|isbn=9780671217150|language=en}}</ref>
*The [[Religion:Four Horsemen of the Apocalypse|Four Horsemen of the Apocalypse]] ride in the [[Religion:Book of Revelation|Book of Revelation]]. ([[Religion:Christianity|Christianity]])<ref>{{Cite book|last1=Wagner|first1=Richard|url=https://books.google.com/books?id=hVpcKP9kxLgC&q=Four+Horsemen+of+the+Apocalypse+ride+in+the+Book+of+Revelation&pg=PA308|title=The Book of Revelation For Dummies|last2=Helyer|first2=Larry R.|date=2011-01-31|publisher=John Wiley & Sons|isbn=978-1-118-05086-6|pages=308|language=en|quote=The four horsemen of the Apocalypse are one of the most familiar images of Revelation}}</ref>
*The four holy cities of Judaism: [[Place:Jerusalem|Jerusalem]], [[Place:Hebron|Hebron]], Safed, and Tiberius<ref>{{Cite book|last=Turfe|first=Tallal Alie|url=https://books.google.com/books?id=T7UOtcZlYWwC&q=Four+Holy+Cities+of+Judaism&pg=PA91|title=Children of Abraham: United We Prevail, Divided We Fail|date=2013-07-19|publisher=iUniverse|isbn=978-1-4759-9047-8|pages=91|language=en|quote=The four holy cities of Judaism are Jerusalem, Hebron, Safed, and Tiberius.}}</ref>

===Hinduism===
*There are four [[Religion:Vedas|Vedas]]: Rigveda, Samaveda, Yajurveda and Atharvaveda.<ref>{{Cite book|last=Frawley|first=David|url=https://books.google.com/books?id=2rvQCgAAQBAJ&q=four+Vedas:+Rigveda,&pg=PT359|title=Vedic Yoga: The Path of the Rishi|date=2014-10-07|publisher=Lotus Press|isbn=978-0-940676-25-1|language=en|quote=There are four Vedas}}</ref>
*In [[Philosophy:Puruṣārtha|Puruṣārtha]], there are four aims of human life: [[Philosophy:Dharma|Dharma]], [[Philosophy:Artha|Artha]], Kāma, [[Philosophy:Moksha|Moksha]].<ref>{{Cite book|last=Fritz|first=Stephen Martin|url=https://books.google.com/books?id=HaeXDwAAQBAJ&q=Puru&pg=PA491|title=Our Human Herds: The Theory of Dual Morality (Second Edition, Unabridged)|date=2019-05-14|publisher=Dog Ear Publishing|isbn=978-1-4575-6755-1|pages=491|language=en|quote=that these four proper aims and objects}}</ref>
*The four stages of life [[Philosophy:Brahmacharya|Brahmacharya]] (student life), [[Philosophy:Grihastha|Grihastha]] (household life), Vanaprastha (retired life) and [[Philosophy:Sannyasa|Sannyasa]] (renunciation).<ref>{{Cite book|url=https://books.google.com/books?id=uC6-6seQycgC&q=four+stages+of+life+Brahmacharya&pg=PT144|title=Maanas - Individual and Society|publisher=Rapid Publications|isbn=978-1-937192-06-8|language=en|quote=The Four Stages of Life}}</ref>
*The four primary castes or strata of society: [[Religion:Brahmana|Brahmana]] (priest/teacher), Kshatriya (warrior/politician), Vaishya (landowner/entrepreneur) and Shudra (servant/manual laborer).<ref>{{Cite book|last=Chwalkowski|first=Farrin|url=https://books.google.com/books?id=W-22DQAAQBAJ&q=four+primary+castes+or+strata+of+society&pg=PA23|title=Symbols in Arts, Religion and Culture: The Soul of Nature|date=2016-12-14|publisher=Cambridge Scholars Publishing|isbn=978-1-4438-5728-4|pages=23|language=en|quote=The four primary castes or strata of society:...}}</ref>
*The [[Swastika|swastika]] symbol is traditionally used in Hindu religions as a sign of good luck and signifies good from all four directions.<ref>{{Cite book|last=Kulendiren|first=Pon|url=https://books.google.com/books?id=mq9UAVT9FqcC&q=swastika+good+from+all+four+directions&pg=PA31|title=Hinduism a Scientific Religion: & Some Temples in Sri Lanka|date=2012-10-11|publisher=iUniverse|isbn=978-1-4759-3675-9|pages=32|language=en}}</ref>
*The god [[Religion:Brahma|Brahma]] has four faces.<ref>{{Cite book|last=Jansen|first=Eva Rudy|url=https://books.google.com/books?id=1iASyoae8cMC&q=Brahma+has+four+faces&pg=PA87|title=The Book of Hindu Imagery: Gods, Manifestations and Their Meaning|date=1993|publisher=Binkey Kok Publications|isbn=978-90-74597-07-4|pages=87|language=en|quote=Brahma has four faces,...}}</ref>
*There are four ''[[Physics:Yuga|yuga]]s'': ''Satya'', ''Dvapara'', ''Treta'' and ''Kali''<ref>{{Cite web|title=Definition of yuga|website=Dictionary.com|url=https://www.dictionary.com/browse/yuga|access-date=2020-07-28|language=en|archive-date=28 July 2020|archive-url=https://web.archive.org/web/20200728214059/https://www.dictionary.com/browse/yuga|url-status=live}}</ref>

===Islam===
*[[Religion:Eid al-Adha|Eid al-Adha]] lasts for four days, from the 10th to the 14th of Dhul Hijja.<ref>{{Cite book|last=Çakmak|first=Cenap|url=https://books.google.com/books?id=JSHFDgAAQBAJ&q=Eid+al-Adha+lasts+for+four+days&pg=PA397|title=Islam: A Worldwide Encyclopedia [4 volumes]|date=2017-05-18|publisher=ABC-CLIO|isbn=978-1-61069-217-5|pages=397|language=en|quote=...Eid al-Adha (Feast of Sacrifice) lasts four days ...}}</ref>
*The four holy cities of Islam: Mecca, Medina, Jerusalem and Damascus.
*The four tombs in the Green Dome: Muhammad, Abu Bakr, Umar ibn Khattab and Isa ibn Maryam (Jesus).
*There are four [[Religion:Rashidun|Rashidun]] or Rightly Guided Caliphs: Abu Bakr, Umar ibn al-Khattab, Uthman ibn Affan and Ali ibn Abi Talib.<ref>{{Cite book|last1=Leonard|first1=Timothy|url=https://books.google.com/books?id=FBCnQx-kaK8C&q=our+Rashidun+or+Rightly+Guided+Caliphs:+Abu+Bakr,+Umar+ibn+al-Khattab,+Uthman+ibn+Affan+and+Ali+ibn+Abi+Talib.&pg=PA144|title=Pedagogies of the Imagination: Mythopoetic Curriculum in Educational Practice|last2=Willis|first2=Peter|date=2008-06-11|publisher=Springer Science & Business Media|isbn=978-1-4020-8350-1|pages=144|language=en|quote=... four Rightly Guided Caliphs, Abu-Bakr, Umar ibn al-Khattab, Uthman ibn Affan and Ali ibn Abi Talib,...}}</ref>
*The Four Arch Angels in Islam are: Jibraeel (Gabriel), Mikaeel (Michael), Izraeel (Azrael), and Israfil (Raphael)<ref>{{Cite book|last=Chwalkowski|first=Farrin|url=https://books.google.com/books?id=W-22DQAAQBAJ&q=Four+Arch+Angels+in+Islam+are:+Jibraeel+(Gabriel),+Mikaeel+(Michael),+Izraeel+(Azrael),+and+Israfil+(Raphael)&pg=PA23|title=Symbols in Arts, Religion and Culture: The Soul of Nature|date=2016-12-14|publisher=Cambridge Scholars Publishing|isbn=978-1-4438-5728-4|pages=23|language=en|quote=According to Islam, the Four Arch Angels are: Jibraeel (Gabriel), Mikaeel (Michael), Izraeel (Azrael), and Israfil (Raphael).}}</ref>
*There are four months in which war is not permitted: [[Religion:Muharram|Muharram]], [[Religion:Rajab|Rajab]], Dhu al-Qi'dah and [[Religion:Dhu al-Hijjah|Dhu al-Hijjah]].<ref>{{Cite book|last=Busool|first=Assad Nimer|url=https://books.google.com/books?id=JmTifTwgaUIC&q=four+months+in+which+war+is+not+permitted:+Muharram,+Rajab,+Dhu+al-Qi'dah+and+Dhu+al-Hijjah.&pg=PA50|title=The Wise Qur'an: These are the Verses of the Wise Book: These are the verses of the Wise Book|date=2010-12-28|publisher=Xlibris Corporation|isbn=978-1-4535-2526-5|pages=50|language=en|quote=The sacred months are four, Rajab, Dhu al-Qi'dah, Dhu al-Hijjah, and al-Muharram. During those four sacred months there were no war...}}</ref>
*There are four Sunni schools of [[Religion:Fiqh|fiqh]]: Hanafi, Shafi`i, Maliki and Hanbali.
*There are four major Sunni Imams: Abū Ḥanīfa, Muhammad ibn Idris ash-Shafi`i, Malik ibn Anas and Ahmad ibn Hanbal.
*There are four [[Religion:Islamic holy books|books in Islam]]: Taurāt, Zābūr, Injīl, Qur'ān.<ref>{{Cite book|last=Shabazz|first=Hassan|url=https://books.google.com/books?id=9W3NDwAAQBAJ&q=four+books+in+Islam:+Torah,+Zaboor,+Injeel,+Quran&pg=PA15|title=Al Islaam, and the Transformation of Society|date=2020-01-06|publisher=Lulu.com|isbn=978-1-7948-3337-1|pages=15|language=en|quote=There are four books in Islam: Torah, Zaboor, Injeel and Holy Qur'an...}}</ref>
*Waiting for four months is ordained for those who take an oath for abstention from their wives.<ref>{{Cite book|last=Bukhari|first=Muohammad Ben Ismail Al|url=https://books.google.com/books?id=HcNtDwAAQBAJ&q=four+months+is+ordained+for+those+who+take+an+oath+for+abstention&pg=PA840|title=THE CORRECT TRADITIONS OF AL'BUKHARI 1-4 VOL 3: صحيح البخاري 1/4 [عربي/انكليزي] ج3|date=2007-01-01|publisher=Dar Al Kotob Al Ilmiyah دار الكتب العلمية|pages=840|language=en|quote=For those who take an oath for abstention from their wives, awaiting for four months is ordained;}}</ref>
*The waiting period of the woman whose husband dies is [[Religion:Islamic funeral#Directives for widows|four months and ten days]].<ref>{{Cite book|last=Ahmad|first=Yusuf Al-Hajj|url=https://books.google.com/books?id=QD3DAwAAQBAJ&q=waiting+period+of+the+woman+whose+husband+dies,+is+four+months+and+ten+days&pg=PT42|title=The Book Of Nikkah: Encyclopaedia of Islamic Law|publisher=Darussalam Publishers|language=en|quote=...for four months and ten days.}}</ref>
*When Abraham said: "My Lord, show me how You give life to the dead," Allah said: "Why! Do you have no faith?" Abraham replied: "Yes, but in order that my heart be at rest." He said: "Then take four birds, and tame them to yourself, then put a part of them on every hill, and summon them; they will come to you flying. [Al-Baqara 2:260]<ref>{{Cite book|last=Mawdudi|first=Sayyid Abul A'la|url=https://books.google.com/books?id=qkarDQAAQBAJ&q=Then+take+four+birds,+and+tame+them&pg=PA59|title=Towards Understanding the Qur'an: English Only Edition|date=2016-12-15|publisher=Kube Publishing Ltd|isbn=978-0-86037-613-2|pages=59|language=en|quote=Then take four birds, ...}}</ref>
*The respite of four months was granted to give time to the mushriks in Surah At-Tawba so that they should consider their position carefully and decide whether to make preparation for war or to emigrate from the country or to accept Islam.<ref>{{Cite book|last=Maudoodi|first=Syed Abul ʻAla|url=https://books.google.com/books?id=onkkAQAAIAAJ&q=respite+of+four+months+was+granted+to+give+time+to+the+mushriks|title=Sūrah al-Aʻarāf to Sūrah bani Isrāel|date=2000|publisher=Islamic Publications|pages=177|language=en|quote=The respite of four months...}}</ref>
*Those who accuse honorable women (of unchastity) but do not produce four witnesses, flog them with eighty lashes, and do not admit their testimony ever after. They are indeed transgressors. [An-Noor 24:4]<ref>{{Cite book|last=Barazangi|first=Nimat Hafez|url=https://books.google.com/books?id=cdK1CwAAQBAJ&q=honorable+women+(of+unchastity)+but+do+not+produce+four+witnesses&pg=PA138|title=Woman's Identity and Rethinking the Hadith|date=2016-03-09|publisher=Routledge|isbn=978-1-134-77065-6|pages=138|language=en|quote=And those who launch a charge against chaste women and do not produce four witnesses...}}</ref>

===Taoism===
*[[Astronomy:Four Symbols|Four Symbols]] of [[Philosophy:I Ching|I Ching]]<ref>{{Cite book|last=SK|first=Lim|url=https://books.google.com/books?id=pc7_DAAAQBAJ&q=Four+Symbols+Taoism&pg=PA16|title=Origins of Chinese Auspicious Symbols|publisher=Asiapac Books Pte Ltd|isbn=978-981-317-026-1|pages=16|language=en|quote=Taoism later incorporated the four symbols into its immortality system...}}</ref>

===Other===
*In a more general sense, numerous mythological and cosmogonical systems consider [[Four corners of the world]] as essentially corresponding to the four points of the compass.<ref>{{Cite book|last=Terry|first=Milton Spenser|url=https://books.google.com/books?id=URNNAQAAMAAJ&q=Four+corners+of+the+world+as+essentially+corresponding+to+the+four+points+of+the+compass.&pg=PA382|title=Biblical Hermenutics: A Treatise on the Interpretation of the Old and New Testaments|date=1883|publisher=Phillips & Hunt|pages=382|language=en|quote=the four corners or extremities of the earth (Isa. xi, 12; Ezek. vii, 2.; Rev. vii, 1; xx, 8), corresponding, doubtless, with the four points of the compass}}</ref>
*Four is the sacred number of the Zia, an indigenous tribe located in the U.S. state of New Mexico.<ref>{{Cite book|url=https://books.google.com/books?id=GZpLAQAAMAAJ&q=Four+is+the+sacred+number+of+the+Zia|title=Bulletin - State Department of Education|date=1955|publisher=Department of Education|pages=151|language=en|quote=Four was a sacred number of Zia}}</ref>
*The Chinese, the Koreans, and the Japanese are [[Unsolved:Tetraphobia|superstitious about the number four]] because it is a homonym for "death" in their languages.<ref>{{Cite book|last=Lachenmeyer|first=Nathaniel|url=https://books.google.com/books?id=sDXJ1s0YNAgC&q=+homonym+for+%22death%22|title=13: The Story of the World's Most Notorious Superstition|date=2005|publisher=Penguin Group (USA) Incorporated|isbn=978-0-452-28496-8|pages=187|language=en|quote=In Chinese, Japanese, and Korean, the word for four is, unfortunately, an exact homonym for death}}</ref>
*In Slavic mythology, the god Svetovid has four heads.<ref>{{Cite book|last1=Maberry|first1=Jonathan|url=https://books.google.com/books?id=88q1hJk0gjQC&q=Svetovid+has+four+heads.&pg=PA211|title=The Cryptopedia: A Dictionary of the Weird, Strange & Downright Bizarre|last2=Kramer|first2=David F.|date=2007|publisher=Citadel Press|isbn=978-0-8065-2819-9|pages=211|language=en|quote=Svetovid is portrayed as having four heads ...}}</ref>

==In politics==
*Four Freedoms: four fundamental freedoms that Franklin D. Roosevelt declared ought to be enjoyed by everyone in the world: Freedom of Speech, Freedom of Religion, Freedom from Want, Freedom from Fear.<ref>{{Cite web|title=FDR, 'The Four Freedoms,' Speech Text|url=https://voicesofdemocracy.umd.edu/fdr-the-four-freedoms-speech-text/|access-date=2020-07-30|website=Voices of Democracy|language=en-US|archive-date=1 August 2020|archive-url=https://web.archive.org/web/20200801005700/http://voicesofdemocracy.umd.edu/fdr-the-four-freedoms-speech-text/|url-status=live}}</ref>
*Gang of Four: Popular name for four Chinese Communist Party leaders who rose to prominence during China's Cultural Revolution, but were ousted in 1976 following the death of Chairman [[Biography:Mao Zedong|Mao Zedong]]. Among the four was Mao's widow, [[Biography:Jiang Qing|Jiang Qing]]. Since then, many other political factions headed by four people have been called "Gangs of Four".<ref>{{Cite news|title=Yao Wenyuan|newspaper=The Economist|url=https://www.economist.com/obituary/2006/01/12/yao-wenyuan|access-date=2020-07-30|issn=0013-0613|archive-date=1 May 2018|archive-url=https://web.archive.org/web/20180501231531/https://www.economist.com/node/5381892|url-status=live}}</ref>

==In computing==
*Four [[Bit|bit]]s (half a [[Byte|byte]]) are sometimes called a [[Nibble|nibble]].<ref name="Intel_1974_MCS-40">{{cite book |title=MCS-40 User's Manual For Logic Designers |chapter=The Functions Of A Computer: Instruction Register And Decoder |publisher=Intel Corporation |editor-first=Howard A. |editor-last=Raphael |date=November 1974 |location=Santa Clara, California, USA |page=viii |chapter-url=http://bitsavers.trailing-edge.com/components/intel/MCS40/MCS-40_Users_Manual_Nov74.pdf |access-date=2020-03-03 |url-status=live |archive-url=https://web.archive.org/web/20200303024244/http://bitsavers.trailing-edge.com/components/intel/MCS40/MCS-40_Users_Manual_Nov74.pdf |archive-date=2020-03-03 |quote=[...] The characteristic eight [[Bit|bit]] field is sometimes referred to as a [[Byte|byte]], a four bit field can be referred to as a nibble. [...]}}</ref>

==In science==
*A tetramer is an [[Chemistry:Oligomer|oligomer]] formed out of four sub-units.<ref>{{Cite book|last1=Petsko|first1=Gregory A.|url=https://books.google.com/books?id=bCI5u_19N_oC&q=tetramer++four+sub-units+oligomer&pg=PA40|title=Protein Structure and Function|last2=Ringe|first2=Dagmar|date=2004|publisher=New Science Press|isbn=978-0-87893-663-2|pages=40|language=en|quote=Oligomers containing two, three, four, five, six or even more subunits are known as dimers, trimers, tetramers, pentamers, hexamers, and so on.}}</ref>

===In astronomy===
*Four terrestrial (or rocky) planets in the [[Astronomy:Solar System|Solar System]]: [[Astronomy:Mercury (planet)|Mercury]], [[Astronomy:Venus|Venus]], [[Astronomy:Earth|Earth]], and [[Astronomy:Mars|Mars]].<ref>{{Cite book|last=Yaqoob|first=Tahir|url=https://books.google.com/books?id=QhaEz5p0TccC&q=Four+terrestrial+(or+rocky)+planets+in+the+Solar+system:+Mercury,+Venus,+Earth,+and+Mars.&pg=PA12|title=Exoplanets and Alien Solar Systems| date=2011|publisher=New Earth Labs|isbn=978-0-9741689-2-0|pages=12|language=en|quote=The four inner planets (known as terrestrial, or rocky planets}}</ref>
*Four giant gas/ice planets in the [[Astronomy:Solar System|Solar System]]: [[Astronomy:Jupiter|Jupiter]], [[Astronomy:Saturn|Saturn]], [[Astronomy:Uranus|Uranus]], and [[Astronomy:Neptune|Neptune]].<ref>{{Cite book|last1=Encrenaz|first1=Therese|url=https://books.google.com/books?id=Je61Y7UbqWgC&q=Four+giant+gas/ice+planets+in+the+Solar+system:+Jupiter,+Saturn,+Uranus,+and+Neptune&pg=PA283|title=The Solar System|last2=Bibring|first2=Jean-Pierre|last3=Blanc|first3=M.|last4=Barucci|first4=Maria-Antonietta|last5=Roques|first5=Francoise|last6=Zarka|first6=Philippe|date=2004-01-26|publisher=Springer Science & Business Media|isbn=978-3-540-00241-3|pages=283|language=en|quote=...the gas giants (Jupiter and Saturn), and the icy giants (Uranus and Neptune)|access-date=4 November 2020|archive-date=12 March 2022|archive-url=https://web.archive.org/web/20220312050454/https://books.google.com/books?id=Je61Y7UbqWgC&q=Four+giant+gas/ice+planets+in+the+Solar+system:+Jupiter,+Saturn,+Uranus,+and+Neptune&pg=PA283|url-status=live}}</ref>
*Four of Jupiter's moons (the [[Astronomy:Galilean moons|Galilean moons]]) are readily visible from Earth with a hobby telescope.<ref>{{Cite book|last=Pidwirny|first=Michael|url=https://books.google.com/books?id=J7ViAwAAQBAJ&q=Four+of+Jupiter's+moons+(the+Galilean+moons)+are+readily+visible+from+Earth&pg=PA10|title=Chapter 3: Matter, Energy and the Universe: Single chapter from the eBook Understanding Physical Geography|date=2020-05-07|publisher=Our Planet Earth Publishing|pages=10|language=en|quote=including the four large Galilean moons that are easily visible from a hobby telescope}}</ref>
*[[Astronomy:Messier object|Messier object]] [[Astronomy:Messier 4|M4]], a magnitude 7.5 [[Astronomy:Globular cluster|globular cluster]] in the constellation [[Astronomy:Scorpius|Scorpius]].<ref>{{Cite book|last=Pugh|first=Philip|url=https://books.google.com/books?id=jDbjx-FSv5cC&q=Messier+object+M4&pg=PA41|title=Observing the Messier Objects with a Small Telescope: In the Footsteps of a Great Observer|date=2011-11-02|publisher=Springer Science & Business Media|isbn=978-0-387-85357-4|pages=41|language=en|quote=M4 is a globular star cluster near Antares in Scorpius.}}</ref>
*The Roman numeral IV stands for [[Astronomy:Subgiant|subgiant]] in the [[Astronomy:Stellar classification|Yerkes spectral classification scheme]].<ref>{{Cite book|last1=Bok|first1=Bart Jan|url=https://books.google.com/books?id=ea7mzrgKHG8C&q=IV+stands+for+subgiant+in+the+Yerkes+spectral+classification+scheme.&pg=PA66|title=The Milky Way|last2=Bok|first2=Priscilla Fairfield|date=1981|publisher=Harvard University Press|isbn=978-0-674-57503-5|pages=66|language=en|quote=IV, subgiants}}</ref>

===In biology===
*Four is the number of [[Biology:Nucleobase|nucleobase]] types in [[Biology:DNA|DNA]] and [[Biology:RNA|RNA]] – [[Chemistry:Adenine|adenine]], [[Chemistry:Guanine|guanine]], [[Chemistry:Cytosine|cytosine]], [[Chemistry:Thymine|thymine]] ([[Chemistry:Uracil|uracil]] in [[Biology:RNA|RNA]]).<ref>{{Cite book|url=https://books.google.com/books?id=BEabBwAAQBAJ&q=nucleobase+types+in+DNA+and+RNA+%E2%80%93+adenine,+guanine,+cytosine,+thymine&pg=PA25|title=Encyclopedia of Cell Biology|date=2015-08-07|publisher=Academic Press|isbn=978-0-12-394796-3|pages=25|language=en}}</ref>
*Many [[Biology:Chordate|chordate]]s have four feet, legs or leglike appendages (tetrapods).
*The [[Biology:Mammal|mammal]]ian [[Biology:Heart|heart]] consists of four chambers.<ref>{{Cite book|last1=Chien|first1=Shu|url=https://books.google.com/books?id=qp8tO28RGfIC&q=mammalian+heart+consists+of+four+chambers&pg=PA54|title=An Introductory Text to Bioengineering|last2=Chen|first2=Peter C. Y.|last3=Fung|first3=Yuan-cheng|date=2008|publisher=World Scientific|isbn=978-981-270-793-2|pages=54|language=en|quote=The mammalian heart consists of four chambers,...}}</ref>
*Many [[Biology:Mammal|mammal]]s ([[Biology:Carnivora|Carnivora]], Ungulata) use four fingers for movement.
*All [[Biology:Insect|insect]]s with wings except [[Biology:Fly|flies]] and some others have four [[Physics:Wing|wing]]s.<ref>{{Cite book|author=Creation Research Society Textbook Committee|url=https://books.google.com/books?id=MoVQ-izvM5oC&q=insects+with+wings+except+flies+have+four+wings.|title=Biology: a search for order in complexity|date=1970|publisher=Zondervan Pub. House|isbn=978-0-310-29490-0|pages=209|language=en|quote=Except for the flies, mosquitoes, and some others, insects with wings have four wings.}}</ref>
*Insects of the superorder [[Biology:Holometabola|Holometabola]], also known as Endopterygota, such as butterflies, ants, bees, beetles, fleas, flies, moths, and wasps, undergo [[Biology:Holometabolism|holometabolism]]—complete metamorphism in four stages—from (1) embryo (ovum, egg), to (2) larva (such as grub, caterpillar), then (3) pupa (such as the chrysalis), and finally (4) the imago.<ref>{{Cite book|last=Pittenger|first=Dennis|url=https://books.google.com/books?id=yZhoBgAAQBAJ&q=complete+metamorphism+in+four+stages&pg=PA180|title=California Master Gardener Handbook, 2nd Edition|date=2014-12-15|publisher=UCANR Publications|isbn=978-1-60107-857-5|pages=180|language=en|quote=metamorphosis is marked by four distinct stages}}</ref>
*In the common [[Biology:ABO blood group system|ABO blood group system]], there are four blood types (A, B, O, AB).<ref>{{Cite book|last=Darpan|first=Pratiyogita|url=https://books.google.com/books?id=xegDAAAAMBAJ&q=ABO+blood+group+system,+there+are+4+blood+types+(A,+B,+O,+AB)&pg=PT84|title=Pratiyogita Darpan|date=2008|publisher=Pratiyogita Darpan|pages=85|language=en|quote=In the 'ABO' system, all blood belongs one of four major groups — A, B, AB or O}}</ref>
*[[Humans]] have four [[Biology:Canine tooth|canines]] and four wisdom teeth.<ref>{{Cite book|last1=Daniels|first1=Patricia|url=https://books.google.com/books?id=xwmcH-YIJMkC&q=Humans+have+four+canines+and+four+wisdom+teeth.&pg=PA94|title=Body: The Complete Human : how it Grows, how it Works, and how to Keep it Healthy and Strong|last2=Stein|first2=Lisa|date=2009|publisher=National Geographic Books|isbn=978-1-4262-0449-4|pages=94|language=en|quote=Four canines for tearing + Eight premolars for crushing +Twelve molars (including four wisdom teeth)}}</ref>
*The cow's stomach is divided in four digestive compartments: reticulum, rumen, omasum and abomasum.<ref>{{Cite book|last1=Woodward|first1=Thompson Elwyn|url=https://books.google.com/books?id=DAHYktKQo1wC&q=cow's+stomach+is+divided+in+four+digestive+compartments|title=Feeding Dairy Cows|last2=Nystrom|first2=Amer Benjamin|date=1930|publisher=U.S. Department of Agriculture|pages=4|language=en|quote=The cow's stomach is divided into four compartments.}}</ref>

===In chemistry===
*[[Chemistry:Valence|Valency]] of [[Chemistry:Carbon|carbon]] (that is basis of life on the Earth) is four. Also because of its tetrahedral crystal bond structure, [[Chemistry:Diamond|diamond]] (one of the natural allotropes of carbon) is the hardest known naturally occurring material. It is also the valence of [[Chemistry:Silicon|silicon]], whose compounds form the majority of the mass of the Earth's crust.<ref>{{Cite book|last=Lucas|first=Jerry|url=https://books.google.com/books?id=LiEFoyu8iWUC&q=Valency+of+carbon+(that+is+basis+of+life+on+the+Earth)+is+four.|title=Great unsolved mysteries of science|date=1993|publisher=F & W Pubns Inc|isbn=978-1-55870-291-2|pages=168|language=en|quote=Of course, carbon is not the only chemical element with a valence of +4 or -4}}</ref>
*The [[Physics:Atomic number|atomic number]] of [[Chemistry:Beryllium|beryllium]]<ref>{{Cite book|last=Walsh|first=Kenneth A.|url=https://books.google.com/books?id=3-GbhmSfyeYC&q=atomic+number+of+beryllium&pg=PA93|title=Beryllium Chemistry and Processing|date=2009-01-01|publisher=ASM International|isbn=978-0-87170-721-5|pages=93|language=en|quote=Beryllium has an atomic number of four}}</ref>
*There are four basic states of [[Physics:Matter|matter]]: [[Physics:Solid|solid]], [[Physics:Liquid|liquid]], [[Physics:Gas|gas]], and [[Physics:Plasma|plasma]].<ref>{{Cite book|last1=Ebeling|first1=Werner|url=https://books.google.com/books?id=fsJADwAAQBAJ&q=four+basic+states+of+matter&pg=PA39|title=Quantum Statistics of Dense Gases and Nonideal Plasmas|last2=Fortov|first2=Vladimir E.|last3=Filinov|first3=Vladimir|date=2017-11-27|publisher=Springer|isbn=978-3-319-66637-2|pages=39|language=en|quote=Plasma is one of the four fundamental states of matter, the others being solid, liquid, and gas.}}</ref>

===In physics===
*[[Physics:Special relativity|Special relativity]] and [[Physics:General relativity|general relativity]] treat nature as four-[[Physics:Dimension|dimension]]al: 3D regular [[Space|space]] and one-dimensional [[Time|time]] are treated together and called [[Physics:Spacetime|spacetime]].<ref>{{Cite book|last=Petkov|first=Vesselin|url=https://books.google.com/books?id=AzfFo6A94WEC&q=nature+as+four-dimensional&pg=PA124|title=Relativity and the Nature of Spacetime|date=2009-06-23|publisher=Springer Science & Business Media|isbn=978-3-642-01962-3|pages=124|language=en|quote=should be regarded as a four-dimensional world}}</ref> Also, any event ''E'' has a [[Astronomy:Light cone|light cone]] composed of four zones of possible communication and cause and effect (outside the light cone is strictly incommunicado).
*There are four [[Physics:Fundamental forces|fundamental forces]] ([[Physics:Electromagnetism|electromagnetism]], gravitation, the weak nuclear force, and the [[Physics:Strong nuclear force|strong nuclear force]]).<ref>{{Cite book|last=Giordano|first=Nicholas|url=https://books.google.com/books?id=BwistUlpZ7cC&q=f+four+fundamental+forces&pg=PA1073|title=College Physics: Reasoning and Relationships|date=2009-02-13|publisher=Cengage Learning|isbn=978-0-534-42471-8|pages=1073|language=en|quote=We have referred to the four fundamental forces in nature,...}}</ref>
*In [[Physics:Statistical mechanics|statistical mechanics]], the four functions inequality is an inequality for four functions on a finite [[Distributive lattice|distributive lattice]].<ref>{{Cite book|last1=Alon|first1=Noga|url=https://books.google.com/books?id=XLwRrn4rpk4C&q=Ahlswede%E2%80%93Daykin+inequality+four+functions&pg=PT86|title=The Probabilistic Method|last2=Spencer|first2=Joel H.|date=2011-09-20|publisher=John Wiley & Sons|isbn=978-1-118-21044-4|pages=6.1|language=en|quote=The Four Functions Theorem of Ahlswede Daykin}}</ref>

==In logic and philosophy==
[[File:Mugs of tea viewed from above.jpg|thumb|Four mugs]]
*The symbolic meanings of the number four are linked to those of the cross and the square. "Almost from prehistoric times, the number four was employed to signify what was solid, what could be touched and felt. Its relationship to the cross (four points) made it an outstanding symbol of wholeness and universality, a symbol which drew all to itself". Where lines of latitude and longitude intersect, they divide the earth into four proportions. Throughout the world kings and chieftains have been called "lord of the four suns" or "lord of the four quarters of the earth",<ref>Chevalier, Jean and Gheerbrant, Alain (1994), ''The Dictionary of Symbols''. The quote beginning "Almost from prehistoric times..." is on p. 402.</ref> which is understood to refer to the extent of their powers both territorially and in terms of total control of their subjects' doings. 
*The Square of Opposition, in both its Aristotelian version and its Boolean version, consists of four forms: A ("All ''S'' is ''R''"), I ("Some ''S'' is ''R''"), E ("No ''S'' is ''R''"), and O ("Some ''S'' is not ''R''").
*In regard to whether two given propositions can have the same [[Philosophy:Truth value|truth value]], there are four separate logical possibilities: the propositions are ''subalterns'' (possibly both are true, and possibly both are false); ''subcontraries'' (both may be true, but not that both are false); ''contraries'' (both may be false, but not that both are true); or ''contradictories'' (it is not possible that both are true, and it is not possible that both are false).
*[[Biography:Aristotle|Aristotle]] held that there are basically [[Philosophy:Four causes|four causes]] in nature: the [[Philosophy:Four causes#Material cause|material]], the [[Philosophy:Four causes#Formal cause|formal]], the [[Philosophy:Four causes#Efficient cause|efficient]], and the [[Philosophy:Four causes#Final cause|final]].<ref>{{Cite book|last=Hennig|first=Boris|url=https://books.google.com/books?id=1Q0GvgEACAAJ|title=Aristotle's Four Causes|date=2018-12-05|publisher=Peter Lang|isbn=978-1-4331-5929-9|language=en|quote=This book examines Aristotle's four causes (material, formal, efficient, and final)}}</ref>
*The [[Philosophy:Stoicism|Stoics]] held with four basic categories, all viewed as bodies (substantial and insubstantial): (1) ''substance'' in the sense of substrate, primary formless matter; (2) ''quality'', matter's organization to differentiate and individualize something, and coming down to a physical ingredient such as ''pneuma'', breath; (3) ''somehow holding'' (or ''disposed''), as in a posture, state, shape, size, action, and (4) ''somehow holding'' (or ''disposed'') ''toward something,'' as in relative location, familial relation, and so forth.
*[[Biography:Immanuel Kant|Immanuel Kant]] expounded a [[Philosophy:Category (Kant)#The table of judgments|table of judgments]] involving four three-way alternatives, in regard to (1) Quantity, (2) Quality, (3) Relation, (4) Modality, and, based thereupon, a [[Philosophy:Category (Kant)#The table of categories|table of four categories]], named by the terms just listed, and each with three subcategories.
*[[Biography:Arthur Schopenhauer|Arthur Schopenhauer]]'s doctoral thesis was ''On the Fourfold Root of the Principle of Sufficient Reason''.
*[[Biography:Franz Brentano|Franz Brentano]] held that any major philosophical period has four phases: (1) Creative and rapidly progressing with scientific interest and results; then declining through the remaining phases, (2) practical, (3) increasingly skeptical, and (4) literary, mystical, and scientifically worthless—until philosophy is renewed through a new period's first phase. (See Brentano's essay "The Four Phases of Philosophy and Its Current State" 1895, tr. by Mezei and Smith 1998.)
*[[Biography:Charles Sanders Peirce|C. S. Peirce]], usually a [[Philosophy:Trichotomy|trichotomist]], discussed four methods for overcoming troublesome uncertainties and achieving secure beliefs: (1) the method of tenacity (policy of sticking to initial belief), (2) the method of authority, (3) the method of congruity (following a fashionable [[Philosophy:Paradigm|paradigm]]), and (4) the [[Philosophy:Fallibilism|fallibilistic]], self-correcting method of science (see "The Fixation of Belief", 1877); and four barriers to inquiry, barriers refused by the fallibilist: (1) assertion of absolute certainty; (2) maintaining that something is unknowable; (3) maintaining that something is inexplicable because absolutely basic or ultimate; (4) holding that perfect exactitude is possible, especially such as to quite preclude unusual and anomalous phenomena (see "[https://web.archive.org/web/20120106071421/http://www.princeton.edu/~batke/peirce/frl_99.htm F.R.L.]" [First Rule of Logic], 1899).
*[[Biography:Paul Weiss (philosopher)|Paul Weiss]] built a system involving four modes of being: Actualities (substances in the sense of substantial, spatiotemporally finite beings), Ideality or Possibility (pure normative form), Existence (the dynamic field), and God (unity). (See Weiss's ''Modes of Being'', 1958).
*[[Biography:Karl Popper|Karl Popper]] outlined a tetradic schema to describe the growth of theories and, via generalization, also the emergence of new behaviors and living organisms: (1) problem, (2) tentative theory, (3) (attempted) error-elimination (especially by way of critical discussion), and (4) new problem(s). (See Popper's ''Objective Knowledge'', 1972, revised 1979.)
*John Boyd (military strategist) made his key concept the decision cycle or OODA loop, consisting of four stages: (1) observation (data intake through the senses), (2) orientation (analysis and synthesis of data), (3) decision, and (4) action.<ref>{{Cite book|last=Wilkinson|first=Amy|url=https://books.google.com/books?id=DcwNBAAAQBAJ&q=OODA+loop&pg=PA79|title=The Creator's Code: The Six Essential Skills of Extraordinary Entrepreneurs|date=2015-02-17|publisher=Simon and Schuster|isbn=978-1-4516-6609-0|pages=79|language=en|quote=The OODA loop consists of four steps.}}</ref> Boyd held that his decision cycle has philosophical generality, though for strategists the point remains that, through swift decisions, one can disrupt an opponent's decision cycle.
*[[Biography:Richard McKeon|Richard McKeon]] outlined four classes (each with four subclasses) of modes of philosophical inquiry: (1) Modes of Being (Being); (2) Modes of Thought (That which is); (3) Modes of Fact (Existence); (4) Modes of Simplicity (Experience)—and, corresponding to them, four classes (each with four subclasses) of philosophical semantics: Principles, Methods, Interpretations, and Selections. (See McKeon's "Philosophic Semantics and Philosophic Inquiry" in ''Freedom and History and Other Essays'', 1989.)
*[[Biography:E. J. Lowe (philosopher)|Jonathan Lowe]] (E.J. Lowe) argues in ''The Four-Category Ontology'', 2006, for four categories: ''kinds'' (substantial universals), ''attributes'' (relational universals and property-universals), ''objects'' (substantial particulars), and ''modes'' (relational particulars and property-particulars, also known as "[[Philosophy:Trope|tropes]]"). (See Lowe's "Recent Advances in Metaphysics," 2001, [http://www.cs.vassar.edu/~weltyc/fois/fois-2001/keynote/ Eprint])
*Four opposed camps of the morality and nature of evil: [[Philosophy:Moral absolutism|moral absolutism]], amoralism, [[Philosophy:Moral relativism|moral relativism]], and [[Philosophy:Moral universalism|moral universalism]].

==In technology==
[[File:U+2676 DejaVu Sans.svg|100px|thumb|4 as a resin identification code, used in recycling]]
*The [[Resin identification code|resin identification code]] used in recycling to identify [[Chemistry:Low-density polyethylene|low-density polyethylene]].<ref>{{Cite web|last1=Howard|first1=Brian Clark|last2=Abdelrahman|first2=Amina Lake|author3=Good Housekeeping Institute|date=2020-02-26|title=You Might Be Recycling Wrong — Here's Everything You Need to Know About Recycling Symbols|url=https://www.goodhousekeeping.com/home/green-living/recycling-symbols-plastics-460321|access-date=2020-07-28|website=Good Housekeeping|language=en-US|quote=Plastic Recycling Symbol #4: LDPE|archive-date=13 March 2015|archive-url=https://web.archive.org/web/20150313073239/http://www.goodhousekeeping.com/home/green-living/recycling-symbols-plastics-460321|url-status=live}}</ref>
*Most [[Engineering:Furniture|furniture]] has four legs – tables, chairs, etc.
*The four color process (CMYK) is used for printing.<ref>{{Cite book|last=Conover|first=Charles|url=https://books.google.com/books?id=mx8V3QoUSMIC&q=four+color+printing&pg=PA62|title=Designing for Print|date=2011-11-08|publisher=John Wiley & Sons|isbn=978-1-118-13088-9|pages=62|language=en|quote=CMYK is the standard four-color model used for all full-color print jobs that will be output on an offset printing press}}</ref>
*Wide use of [[Rectangle|rectangle]]s (with four angles and four sides) because they have effective form and capability for close adjacency to each other (houses, rooms, tables, bricks, sheets of paper, screens, film frames).
*In the [[Rich Text Format]] specification, language code 4 is for the [[Social:Chinese language|Chinese language]]. Codes for regional variants of Chinese are congruent to {{nowrap|4 mod 256}}.
*[[Finance:Credit card|Credit card]] machines have four-twelve function keys.
*On most phones, the 4 key is associated with the letters G, H, and I,<ref>{{Cite book|last1=Vermaat|first1=Misty E.|url=https://books.google.com/books?id=a0V-BAAAQBAJ&q=G,+H,+and+I,+phone+keypad&pg=PA123|title=Discovering Computers, Essentials|last2=Sebok|first2=Susan L.|last3=Freund|first3=Steven M.|last4=Campbell|first4=Jennifer T.|last5=Frydenberg|first5=Mark|date=2015-01-01|publisher=Cengage Learning|isbn=978-1-305-53402-5|pages=123|language=en|quote=...the 4 key (labeled with the letters g, h and i)...}}</ref> but on the [[Engineering:BlackBerry Pearl|BlackBerry Pearl]], it is the key for D and F.
*On many [[Computer keyboard|computer keyboard]]s, the "4" key may also be used to type the [[Finance:Dollar sign|dollar sign]] ($) if the shift key is held down.
*It is the number of [[Bit|bit]]s in a [[Nibble|nibble]], equivalent to half a [[Byte|byte]]<ref>{{Cite book|last1=Bunting|first1=Steve|url=https://books.google.com/books?id=V_XPRmaOH60C&q=nibble+byte&pg=PA246|title=EnCase Computer Forensics: The Official EnCE: EnCase?Certified Examiner Study Guide|last2=Wei|first2=William|date=2006-03-06|publisher=John Wiley & Sons|isbn=978-0-7821-4435-2|pages=246|language=en|quote=A byte also contains two 4-bit nibbles...}}</ref>
*In [[Internet slang|internet slang]], "4" can replace the word "for" (as "four" and "for" are pronounced similarly). For example, typing "4u" instead of "for you".
*In [[Leet]]speak, "4" may be used to replace the letter "A".
*The [[Internet protocol suite|TCP/IP stack]] consists of four layers.<ref>{{Cite journal|last=Braden|first=R.|editor-first1=R|editor-last1=Braden|title=Requirements for Internet Hosts - Communication Layers|url=https://tools.ietf.org/html/rfc1122.html|access-date=2020-07-28|website=tools.ietf.org|year=1989|pages=9–10|doi=10.17487/RFC1122|language=en|doi-access=free|archive-date=28 July 2020|archive-url=https://web.archive.org/web/20200728124449/https://tools.ietf.org/html/rfc1122.html|url-status=live}}</ref>

==In transport==
[[File:BKV m 4 jms.svg|100px|right]]
*Many internal combustion engines are called four-stroke engines because they complete one thermodynamic cycle in four distinct steps: Intake, compression, power, and exhaust.
*Most vehicles, including motor vehicles, and particularly cars/automobiles and light commercial vehicles have four road wheels.
*"Quattro", meaning four in the Italian language, is used by [[Company:Audi|Audi]] as a trademark to indicate that [[Engineering:All-wheel drive|all-wheel drive]] (AWD) technologies are used on Audi-branded cars.<ref>{{Cite magazine|last=Assenza|first=Tony|url=https://books.google.com/books?id=LdkDAAAAMBAJ&q=%22Quattro++audi&pg=PA147|title=Audi Quattro: Germany's 4x4 Cruise Missile|magazine=Popular Mechanics|date=June 1982|publisher=Hearst Magazines|language=en}}</ref> The word "Quattro" was initially used by Audi in 1980 in its original 4WD coupé, the [[Engineering:Audi Quattro|Audi Quattro]]. Audi also has a privately held subsidiary company called quattro GmbH.
*List of highways numbered 4

==In sports==
* In the Australian Football League, the top level of Australian rules football, each team is allowed 4 "interchanges" (substitute players), who can be freely substituted at any time, subject to a limit on the total number of substitutions.
*In baseball:
**There are four bases in the game: first base, second base, third base, and home plate; to score a run, an offensive player must complete, in the sequence shown, a circuit of those four bases.
** When a batter receives four pitches that the umpire declares to be "balls" in a single at-bat, a base on balls, informally known as a "walk", is awarded, with the batter sent to first base. 
**For scoring, number 4 is assigned to the second baseman.
**Four is the most runs that can be scored on any single at bat, whereby all three baserunners and the batter score (the most common being via a grand slam).
**The fourth batter in the batting lineup is called the cleanup hitter.
*In basketball, the number four is used to designate the power forward position, often referred to as "the four spot" or "the four".<ref>{{Cite book|last1=Schaller|first1=Bob|url=https://books.google.com/books?id=aBLrDQAAQBAJ&q=power+forward+position+4+spot&pg=PT163|title=The Everything Kids' Basketball Book: The all-time greats, legendary teams, today's superstars - and tips on playing like a pro|last2=Harnish|first2=Dave|date=2009-09-18|publisher=Simon and Schuster|isbn=978-1-4405-0177-7|language=en|quote=Power forward Referred to as the number 4 spot}}</ref>
*In cricket, a four is a specific type of scoring event, whereby the ball crosses the boundary after touching the ground at least one time, scoring four runs. Taking four wickets in four consecutive balls is typically referred to as a double hat trick (two consecutive, overlapping hat tricks).
*In American Football teams get four downs to reach the line of gain. 
*In rowing, a four refers to a [[Engineering:Boat|boat]] for four rowers, with or without coxswain. In rowing nomenclature, 4− represents a coxless four and 4+ represents a coxed four.
*In rugby league:
** A try is worth 4 points.
** One of the two starting centres wears the jersey number 4. (An exception to this rule is the Super League, which uses static squad numbering.)
*In rugby union:
** One of the two starting locks wears the jersey number 4.
** In the standard bonus points system, a point is awarded in the league standings to a team that scores at least 4 tries in a match, regardless of the match result.
* In women’s artistic gymnastics, there are four events/apparatus uneven bars, balance beam, floor exercise, and vault.

==In other fields==
[[File:ICS Four.svg|right|thumb|100px|International maritime signal flag for 4]]

* The phrase "four-letter word" is used to describe many swear words in the [[Social:English language|English language]].<ref>{{Cite web|title=Definition of FOUR-LETTER WORD|url=https://www.merriam-webster.com/dictionary/four-letter+word|access-date=2020-07-28|website=merriam-webster.com|language=en|archive-date=22 August 2016|archive-url=https://web.archive.org/web/20160822110121/http://www.merriam-webster.com/dictionary/four-letter+word|url-status=live}}</ref>
*Four is the only number whose name in English has the same number of letters as its value.
*Four ({{lang|zh|四}}, formal writing: {{lang|zh|肆}}, [[Pinyin|pinyin]] sì) is considered an unlucky number in Chinese, Korean, Vietnamese and Japanese cultures mostly in Eastern Asia because it sounds like the word "death" ({{lang|zh|死}}, [[Pinyin|pinyin]] sǐ). To avoid complaints from people with tetraphobia, many numbered product lines skip the "four": e.g. [[Company:Nokia|Nokia]] cell phones (there was no series beginning with a 4 until the Nokia 4.2), Palm [[Social:Personal digital assistant|PDAs]], etc. Some buildings skip floor 4 or replace the number with the letter "F", particularly in heavily Asian areas. ''See [[Unsolved:Tetraphobia|tetraphobia]]'' and ''Numbers in Chinese culture''.
*In Pythagorean [[Unsolved:Numerology|numerology]] (a pseudocience) the number 4 represents security and stability.
*The number of characters in a canonical four-character idiom.
*In the NATO phonetic alphabet, the digit 4 is called "fower".<ref>{{Cite book|last=Wells|first=J. C.|url=https://books.google.com/books?id=mkdpBAAAQBAJ&q=fower+IPA&pg=PA33|title=Sounds Interesting: Observations on English and General Phonetics|date=2014-09-25|publisher=Cambridge University Press|isbn=978-1-316-12385-0|pages=33|language=en|quote=But one confused re-spelling is fower for 'four'.}}</ref>
*In [[Unsolved:Astrology|astrology]], [[Astronomy:Cancer (constellation)|Cancer]] is the 4th [[Unsolved:Astrological sign|astrological sign]] of the [[Astronomy:Zodiac|Zodiac]].<ref>{{Cite book|last1=Guttman|first1=Ariel|url=https://books.google.com/books?id=nIpZDPy9ll0C&q=astrology+cancer+4th&pg=PA263|title=Mythic Astrology: Archetypal Powers in the Horoscope|last2=Guttman|first2=Gail|last3=Johnson|first3=Kenneth|date=1993|publisher=Llewellyn Worldwide|isbn=978-0-87542-248-0|pages=263|language=en|quote=Sign: Cancer, the fourth Zodiacal Sign}}</ref>
[[File:I-p-c-s org journal.TIF|right|thumb|100px|Playing cards for 4]]
*In Tarot, The Emperor is the fourth trump or Major Arcana card.<ref>{{Cite book|last=Curtiss|first=Harriette A.|url=https://books.google.com/books?id=mfmbsoMcs2IC&q=Tarot,+The+Emperor+4th&pg=PA161|title=The Key to the Universe|date=1996|publisher=Health Research Books|isbn=978-0-7873-1233-6|pages=161|language=en|quote=The 4th Tarot Card is called "The Emperor."}}</ref>
*In ''[[Software:Tetris|Tetris]]'', a game named for the Greek word for 4, every shape in the game is formed of 4 blocks each.<ref>{{Cite book|last1=Weller|first1=David|url=https://books.google.com/books?id=muM9iw_xlOcC&q=tetris+four+blocks&pg=PA383|title=Beginning .NET Game Programming in VB .NET|last2=Lobao|first2=Alexandre Santos|last3=Hatton|first3=Ellen|date=2004-09-20|publisher=Apress|isbn=978-1-4302-0724-5|pages=383|language=en|quote=...tetraminos (the shapes used in Tetris) are all just a collection of four blocks}}</ref>
*4 represents the number of Justices on the Supreme Court of the United States necessary to grant a writ of certiorari (i.e., agree to hear a case; it is one less than the number necessary to render a majority decision) at the court's current size.<ref>{{Cite book|last1=Bardes|first1=Barbara|url=https://books.google.com/books?id=6ubh-K1gBooC&q=writ+of+certiorari++four+justices&pg=PA453|title=American Government and Politics Today: The Essentials 2009 - 2010 Edition|last2=Shelley|first2=Mack|last3=Schmidt|first3=Steffen|date=2008-12-16|publisher=Cengage Learning|isbn=978-0-495-57170-4|pages=453|language=en|quote=The court will not issue a writ unless at least four justices approve of it. This is called the rule of four.}}</ref>
*Number Four is a character in the book series ''Lorien Legacies.''<ref>{{Cite web|date=2011-02-17|title=Movie Projector: 'I Am Number Four' to be No. 1 at holiday weekend box office [Updated]|url=https://latimesblogs.latimes.com/entertainmentnewsbuzz/2011/02/movie-projector-big-mommas-i-am-number-four-unknown.html|access-date=2020-07-28|website=LA Times Blogs - Company Town|language=en-US|archive-date=20 August 2020|archive-url=https://web.archive.org/web/20200820084940/https://latimesblogs.latimes.com/entertainmentnewsbuzz/2011/02/movie-projector-big-mommas-i-am-number-four-unknown.html|url-status=live}}</ref>
*In the performing arts, the fourth wall is an imaginary barrier which separates the audience from the performers, and is "broken" when performers communicate directly to the audience.<ref>{{Cite web|title=fourth wall|url=https://dictionary.cambridge.org/us/dictionary/english/fourth-wall|access-date=2021-11-29|website=dictionary.cambridge.org|language=en-US}}</ref>

==In music==
*In written music, common time is constructed of four beats per measure and a quarter note receives one beat.<ref>{{Cite book|last=Roberts|first=Gareth E.|url=https://books.google.com/books?id=sT-BCwAAQBAJ&q=%22common+time%22++four+beats+per+measure&pg=PA3|title=From Music to Mathematics: Exploring the Connections|date=2016-02-15|publisher=JHU Press|isbn=978-1-4214-1918-3|pages=3|language=en|quote=... called common time and denoted by C, which has four beats per measure}}</ref>
*In popular or modern music, the most common time signature is also founded on four beats, i.e., 4/4 having four quarter note beats.
*The common major scale is built on two sets of four notes (e.g., CDEF, GABC), where the first and last notes create an octave interval (a pair-of-four relationship).
*The interval of a perfect fourth is a foundational element of many genres of music, represented in music theory as the tonic and subdominant relationship. Four is also embodied within the circle of fifths (also known as circle of fourths), which reveals the interval of four in more active harmonic contexts.
*The typical number of movements in a symphony.<ref>{{Cite book|last=Bonds|first=Mark Evan|url=https://books.google.com/books?id=SBJG1NsoreIC&q=typical+number+of+movements+in+a+symphony&pg=PA1|title=Music as Thought: Listening to the Symphony in the Age of Beethoven|date=2009-01-10|publisher=Princeton University Press|isbn=978-1-4008-2739-8|pages=1|language=en|quote=The number, character and sequence of movements in the symphony, moreover, did not stabilize until the 1770s when the familiar format of four movements...}}</ref>
*The number of completed, numbered symphonies by Johannes Brahms.<ref>{{Cite book|last=Frisch|first=Walter|url=https://books.google.com/books?id=gb9rTvaoE1oC&q=Johannes+Brahms++symphonies|title=Brahms: The Four Symphonies|date=2003|publisher=Yale University Press|isbn=978-0-300-09965-2|language=en}}</ref>
*The number of strings on a [[Engineering:Violin|violin]], a [[Engineering:Viola|viola]], a [[Engineering:Cello|cello]], [[Engineering:Double bass|double bass]], a [[Engineering:Cuatro (instrument)|cuatro]], a typical [[Engineering:Bass guitar|bass guitar]], and a [[Engineering:Ukulele|ukulele]], and the number of string pairs on a [[Engineering:Mandolin|mandolin]].
*"Four calling birds" is the gift on the fourth day of Christmas in the carol "The Twelve Days of Christmas".<ref>{{Cite book|last=Brech|first=Lewis|url=https://books.google.com/books?id=BcTq8pnkWJMC&q=%22Four+calling+birds%22&pg=PA26|title=Storybook Advent Carols Collection Songbook|date=2010|publisher=Couples Company, Inc|isbn=978-1-4524-7763-3|pages=26|language=en}}</ref>

==Groups of four==
* Big Four (disambiguation)
*Four basic operations of arithmetic: [[Addition|addition]], [[Subtraction|subtraction]], [[Multiplication|multiplication]], [[Division (mathematics)|division]].<ref>{{Cite book|last1=Wright|first1=Robert J.|url=https://books.google.com/books?id=64RAtmXqaVcC&q=four+basic+operations+of+arithmetic&pg=PA135|title=Developing Number Knowledge: Assessment, Teaching and Intervention with 7-11 year olds|last2=Ellemor-Collins|first2=David|last3=Tabor|first3=Pamela D.|date=2011-11-04|publisher=SAGE|isbn=978-1-4462-5368-7|language=en}}</ref>
*[[Philosophy:Classical element#Classical elements in Greece|Greek classical elements]] ([[Astronomy:Fire|fire]], air, [[Chemistry:Water|water]], [[Astronomy:Earth|earth]]).<ref>{{Cite book|last=Macauley|first=David|url=https://books.google.com/books?id=vP5W2bBAaOQC&q=fire,+air,+water,+earth|title=Elemental Philosophy: Earth, Air, Fire, and Water as Environmental Ideas|date=2010-09-29|publisher=SUNY Press|isbn=978-1-4384-3246-5|language=en}}</ref>
*[[Physics:Season|Four seasons]]: spring, summer, autumn, winter.
*The Four Seasons (disambiguation)
*A [[Physics:Leap year|leap year]] generally occurs every four years.<ref>{{Cite book|last=Brooks|first=Edward|url=https://books.google.com/books?id=-2EXAAAAYAAJ&q=leap+year+every+four+years&pg=PA227|title=Normal Higher Arithmetic Designed for Advanced Classes in Common Schools, Normal Schools, and High Schools, Academics, Etc|date=1876|publisher=Sower|pages=227|language=en|quote=Every year that is divisible by four, except the Centennial years, and every Centennial year divisible by 400, is a leap year...}}</ref>
*Approximately four weeks (4 times 7 days) to a lunar month (synodic month = 29.53 days). Thus the number four is universally an integral part of primitive sacred calendars.
*Four weeks of [[Religion:Advent|Advent]] (and four Advent candles on the Advent wreath).
*Four cardinal directions: [[North|north]], [[South|south]], [[East|east]], [[West|west]].<ref>{{Cite book|last1=Touche|first1=Fred|url=https://books.google.com/books?id=KA1TXZO8e-EC&q=cardinal+directions+north+east&pg=PA48|title=Wilderness Navigation Handbook|last2=Price|first2=Anne|date=2005|publisher=Touche Publishing|isbn=978-0-9732527-0-5|pages=48|language=en|quote=Each of the familiar cardinal directions is equivalent to a particular true bearing: north (0°), east (90°), south (180°), and west (270°)}}</ref>
*Four Temperaments: [[Sanguine|sanguine]], choleric, [[Philosophy:Melancholia|melancholic]], phlegmatic.
*Four Humors: [[Biology:Blood|blood]], yellow bile, black bile, [[Biology:Phlegm|phlegm]].<ref>{{Cite book|last=Roeckelein|first=J. E.|url=https://books.google.com/books?id=1Yn6NZgxvssC&q=Four+Humors&pg=PA235|title=Elsevier's Dictionary of Psychological Theories|date=2006-01-19|publisher=Elsevier|isbn=978-0-08-046064-2|pages=235|language=en|quote=...four substances or humors: blood, yellow bile, black bile and phlegm}}</ref>
*Four Great Ancient Capitals of China.
*Four-corner method.
*Four Asian Tigers, referring to the economies of [[Place:Hong Kong|Hong Kong]], Taiwan, {{wipe|South Korea}}, and [[Place:Singapore|Singapore]]
*Cardinal principles.
*Four cardinal virtues: justice, prudence, temperance, fortitude.
*Four suits of playing cards: hearts, diamonds, clubs, spades.<ref>{{Cite book|last=Medley|first=H. Anthony|url=https://books.google.com/books?id=BriBbQPCiy4C&q=Playing+card+suits&pg=PA6|title=Bridge|date=1997|publisher=Penguin|isbn=978-0-02-861735-0|pages=6|language=en|quote=The four playing card suits, as you probably already know, are spades, hearts, diamonds, and clubs}}</ref>
*Four [[Earth:Nation|nation]]s of the {{wipe|United Kingdom}}: England, Wales, Scotland, Northern Ireland.
*Four [[Place:Provinces of Ireland|provinces of Ireland]]: Munster, Ulster, Leinster, Connacht.
*Four estates: politics, administration, [[Social:Judiciary|judiciary]], journalism. Especially in the expression "Fourth Estate", which means journalism.
*Four Corners is the only location in the United States where four states come together at a single point: Colorado, Utah, New Mexico, and Arizona.
* [[Religion:Four Evangelists|Four Evangelists]] – Matthew, Mark, Luke, and John
* Four [[Religion:Doctor of the Church|Doctors of Western Church]] – Saint Gregory the Great, Saint [[Biography:Ambrose|Ambrose]], Saint Augustine, and Saint [[Biography:Jerome|Jerome]]
* Four Doctors of Eastern Church – Saint John Chrysostom, Saint Basil the Great, and [[Biography:Gregory of Nazianzus|Gregory of Nazianzus]] and Saint Athanasius
* Four [[Astronomy:Galilean moons|Galilean moons]] of Jupiter – [[Astronomy:Io (moon)|Io]], [[Astronomy:Europa (moon)|Europa]], [[Astronomy:Ganymede (moon)|Ganymede]], and [[Astronomy:Callisto (moon)|Callisto]]
* The Gang of Four was a Chinese communist political faction.
* The Fantastic Four: Mr. Fantastic, The Invisible Woman, The Human Torch, The Thing.
* The Teenage Mutant Ninja Turtles: Leonardo, Michelangelo, Donatello, Raphael
* The Beatles were also known as the "Fab Four": John Lennon, Paul McCartney, George Harrison, Ringo Starr.
* Gang of Four is a British post-punk rock band formed in the late 1970s.
* Four rivers in the [[Religion:Garden of Eden|Garden of Eden]] ([[Religion:Book of Genesis|Genesis]] 2:10–14): [[Religion:Pishon|Pishon]] (perhaps the Jaxartes or Syr Darya), [[Religion:Gihon|Gihon]] (perhaps the Oxus or Amu Darya), Hiddekel ([[Place:Tigris|Tigris]]), and P'rat ([[Place:Euphrates|Euphrates]]).
* There are also four years in a single [[Physics:Olympiad|Olympiad]] (duration between the [[Social:Olympic Games|Olympic Games]]). Many major international sports competitions follow this cycle, among them the FIFA World Cup and its women's version, the FIBA World Championships for men and women, and the Rugby World Cup.
* There are four limbs on the [[Biology:Human|human]] body.
* Four Houses of Hogwarts in the Harry Potter series: Gryffindor, Hufflepuff, Ravenclaw, Slytherin.<ref>{{Cite book|last=Baker|first=Felicity|url=https://books.google.com/books?id=6QA-vgAACAAJ|title=Houses of Hogwarts: Cinematic Guide|date=2017|publisher=Scholastic Incorporated|isbn=978-1-338-12861-1|language=en|quote=...the four houses of Hogwarts School of Witchcraft and Wizardry: Gryffindor, Ravenclaw, Hufflepuff, and Slytherin}}</ref>
* Four known continents of the world in the ''A Song of Ice and Fire'' series: Westeros, Essos, Sothoryos, Ulthos.
* Each Grand Prix in [[Company:Nintendo|Nintendo]]'s ''Mario Kart'' series is divided into four cups and each cup is divided into four courses. The Mushroom Cup, Flower Cup, Star Cup, and Special Cup make up the Nitro Grand Prix, while the Shell Cup, Banana Cup, Leaf Cup, and the Lightning Cup make up the Retro Grand Prix.

== See also ==
*List of highways numbered 4

==References==
{{Reflist}}
*Wells, D. ''The Penguin Dictionary of Curious and Interesting Numbers'' London: Penguin Group. (1987): 55–58

==External links==
*[http://www.marijn.org/everything-is-4 Marijn.Org on Why is everything four?]
*[https://web.archive.org/web/20180303094710/http://www.samuel-beckett.net/Penelope/four_symbolism.html A few thoughts on the number four], by Penelope Merritt at samuel-beckett.net
*[http://www.numdic.com/4 The Number 4]
*[http://www.positiveintegers.org/4 The Positive Integer 4]
*[http://primes.utm.edu/curios/page.php/4.html Prime curiosities: 4]


[[Category:Integers]]
[[Category:4 (number)| ]]

{{Sourceattribution|4}}