4

This article discusses the number Four. Four is also the name of a French commune, Four, Isère.

Template:Numbers (digits)
Cardinal	4 four
Ordinal	4th fourth
Numeral system	quaternary
Factorization	$2^{2}$
Divisors	1, 2, 4
Roman numeral	IV or IIII
Prefixes	tetra- (Greek) quad- (Latin)
Binary	100
Octal	4
Duodecimal	4
Hexadecimal	4

4 (four) is the natural number following 3 and preceding 5. Prefixes for 4 include tetra- (Greek) and quad- (Latin).

Evolution of the glyph

Representing 1, 2 and 3 in as many lines as the number represents worked well enough, but with 4 writing four lines began to appear tiresome. The Brahmin Indians simplified 4 by joining its four lines into a cross that looks like our modern plus sign. The Sunga and other Indians would add a horizontal line on top of the numeral, and the Kshatrapa and Pallava evolved the numeral to a point where speed of writing was at best a secondary concern. The Arabs didn't have time for cursive fancy: their 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 off the finishing curve and gradually made the numeral less cursive, ending up with a glyph that could have evolved in a much simpler way than the convoluted route it actually took: by simply taking the Brahmin cross and adding a line to connect its "western" and "northern" ends.

In mathematics

Four is the smallest composite number, its proper divisors being 1 and 2. Four is also a highly composite number. The next highly composite number is six. Four is the second square number, the second centered triangular number.

Four is the only composite number that is equal to the sum of its prime factors. As a consequence of this, it is the smallest Smith number.

It is also a Motzkin number.

In addition, $2+2=2\times 2=2^{2}=4$ . Continuing the pattern in Knuth's up-arrow notation, $2\uparrow \uparrow 2=2\uparrow \uparrow \uparrow 2=4$ , and so on, for any number of up arrows.

A four-sided plane figure is a quadrilateral, sometimes also called a tetragon. A solid figure with four faces is a tetrahedron. The regular tetrahedron is the simplest Platonic solid.

The smallest non-cyclic group has four elements; it is the Klein four-group. Four is also the order of the smallest non-trivial groups that are not simple.

The four-color theorem states that a planar graph (or, equivalently, a flat map of two-dimensional regions such as countries) can be colored using four colors, so that adjacent vertices (or regions) are always different colors. Three colors are not, in general, sufficient to guarantee this.

Lagrange's four-square theorem states that every positive integer can be written as the sum of at most four square numbers. Three are not always sufficient; 7 for instance cannot be written as the sum of three squares.

Four is the number of n-Queens Problem solutions for n = 6.

There are four basic arithmetic operations: addition, subtraction, multiplication and division.

The Cartesian coordinate system for a plane has four quadrants.

A tetrahedron, which can also be called a 3-simplex, has four triangular faces and four vertices.

Four is the first positive non-Fibonacci number.

Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e. $4x=y^{2}-z^{2}$

Four is an all-Harshad number and a semi-meandric number.

In physics

There are four fundamental forces of nature: gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force.

Special relativity and general relativity treats nature as four-dimensional — time and three-dimensional space are treated together and called spacetime.

Alpha particle (helium nucleus) consists of four hadrons.

In chemistry

Valency of carbon (that is basis of life on the Earth) is four. Thanks to its tetrahedral crystal bond structure diamond (one of the natural allotropes of carbon) is the hardest known naturally occurring material.

In biology

Four is the number of nitrogenous base types in DNA and RNA - adenine, guanine, cytosine, thymine.

Many of chordate have four feet, legs or leglike appendages (Tetrapods).

Heart of Mammals consists of four chambers.

A lot of Mammals (Carnivora, Ungulata) use four fingers for movement.

The fourth finger of a human (on the left hand - ring-finger) is moved when the little finger moves.

Butterflies and dragonflies have four wings.

People have four blood groups.

People have four canines, four incisor and four wisdom teeth.

In astronomy

Four internal planets in Solar system: Mercury, Venus, Earth, Mars.

Four giant gas planets in Solar system: Jupiter, Saturn, Uranus, Neptune.

In technology

Most vehicles have four wheels.

Most furniture has four legs - tables, chairs, etc.

Four horses (quadriga) is the maximal number of horses in one row for carriage.

The four color process (CMYK) is used for printing.

Wide use of rectangles (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 other fields

Four is the only English number that, when spelled out, has the number of letters that it names.

Four seasons: winter, spring, summer and autumn.

Four parts of a day: night, morning, day, evening.

Four is the figurative number of corners that the world has: North, South, West, East.

Four (四, formal writing: 肆, pinyin si4) in considered an unlucky number in Chinese and Japanese cultures because it sounds like the word "death" (死 pinyin si3). Due to that, many numbered product lines skip the "four": e.g. Nokia cell phones (there is no series beginning with a 4), Palm PDAs, the Leisure Suit Larry games, etc..

In the NATO phonetic alphabet, the digit 4 is called "fower".

In the sport of cricket, a four is a specific type of scoring event, scoring four runs.

The four Greek classical elements are fire, air, water, and earth.

Usages include:

Tetramorph
Gang of Four
Four Daughters
Four Noes and One Without
Four Modernizations
The bands The Four Tops and Four Non Blondes
Led Zeppelin's fourth album, untitled but commonly referred to as Four Symbols or Led Zeppelin IV. It contains a song called Four Sticks
Four great ancient capitals of China
Four corner method
Four-character idiom
Four cardinal principles
Four Books

Four is:

The number of strokes in the cycle of a four-stroke engine
The atomic number of beryllium
the number of musicians in a quartet
The number of strings on a violin, a viola, a cello and a double bass
The number of fingers that people have in many cartoons
The number of suits of playing cards
The number of teeth of most forks
The number of nations of the United Kingdom: Wales, Scotland, Northern Ireland and England
The number of Noble Truths in the Buddhist faith
In the expression "fourth estate", referring to journalists.
The designation of Interstate 4, a freeway in Florida
Four, an album by the Blues Traveler
The years: A.D. 4, 4 B.C., or 1904

In rugby union, The number of the lock forward, who usually jumps at number 2 in the line-out.
In baseball, four represents the second baseman's position.

When counting in binary with fingers, expressing 4 as 00100, results in the isolation of the middle fingers of one hand, a gesture generally considered rude.