4 (four) is a number, numeral, and glyph. It is the natural number following 3 and preceding 5.

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 6. Four is the second square number, the second centered triangular number.

Four is the only number in the English language for which the number of letters in its name is equal to the number itself.

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, <math> 2 + 2 = 2 \times 2 = 2^2 = 4 </math>. Continuing the pattern in Knuth's up-arrow notation, <math> 2 \uparrow\uparrow 2 = 2 \uparrow\uparrow\uparrow 2 = 4</math>, and so on, for any number of up arrows.

A four-sided plane figure is a quadrilateral (quadrangle) or square, sometimes also called a tetragon. A circle divided by 4 makes right angles. Because of it, four (4) is the base number of plane (mathematics). Four cardinal directions, four seasons, duodecimal system, and vigesimal system are based on four.

A solid figure with four faces is a tetrahedron. The regular tetrahedron is the simplest Platonic solid. A tetrahedron, which can also be called a 3-simplex, has four triangular faces and four vertices.

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.

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. <math> 4x = y^2 - z^2. </math>

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

The Arabic 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 fonts with text figures, 4 usually has a descender, for example,

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.
• The atomic number of beryllium

In biology

Four is the number of nucleobase types in DNA and RNA - adenine, guanine, cytosine, thymine (uracil in RNA).

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

The mammalian heart consists of four chambers.

Many 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.

There are four human blood groups.

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

In astronomy

Four terrestrial (or rocky) planets in the Solar system: Mercury, Venus, Earth, Mars.

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

Four of Jupiter's moons are readily visible from Earth.

Messier object M4, a magnitude 7.5 globular cluster in the constellation Scorpius.

The New General Catalogue object NGC 4, a faint galaxy in the constellation Pisces

The Saros number of the solar eclipse series which began on -2731 May 6 and ended on -1451 June 13. The duration of Saros series 4 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number of the lunar eclipse series which began on -2700 September 3 and ended on -1131 April 2. The duration of Saros series 4 was 1568.6 years, and it contained 88 lunar eclipses.

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 the Rich Text Format specification, language code 4 is for the Chinese language. Codes for regional variants of Chinese are congruent to 4 mod 256.

Credit card machines have four-twelve function keys.

On most phones, the 4 key is associated with the letters G, H, and I, but on the BlackBerry it is the key for D and F.

On computer keyboards, the "4" key may also be used to type the dollar sign ($).

In sports

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

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.

In basketball, four represents the power forward position.

In ice hockey, four was the jersey number worn by Hall of Famers Jean Beliveau and Bobby Orr.

In other fields

See also 4 (disambiguation)

Four can be used as an obscenity in some technologically adept circles, replacing the common word for fornication. This usage derives from the fact that when counting in binary on ones fingers, 4, whose binary representation is 00100₂ is represented by raising the middle finger of one hand, yielding a vulgar gesture. A stronger variation uses the number 132, whose binary representation 0010000100 replicates this gesture on both hands.

Perhaps deriving from this, a "four-letter word" is used to describe most swear words in the English language, as most such terms do indeed possess four letters.

Four (四, formal writing: 肆, pinyin si4) is considered an unlucky number in Chinese, Korean, 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. Some elevators even skip floor 4, particularly in heavily Asian areas.

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

The number of characters in a canonical four-character idiom.

The number of strings on a violin, a viola, a cello and a double bass.

The designation of Interstate 4, a freeway in Florida.

The number of arms possessed by video game villain and cultural icon Goro.

The number of fingers that people have in many animated cartoons.

Four, an album by the Blues Traveler

4, an album by the Foreigner

The number of completed, numbered symphonies by Johannes Brahms

Four is a commune of the Isère département, in France

In Astrology, Cancer is the 4th astrological sign of the Zodiac.

4 is one of the "Lost Numbers" on the television show, Lost, along with 8, 15, 16, 23, and 42.

Groups of four

• Four rules : addition, subtraction, multiplication, division.
• Greek classical elements (fire, air, water, earth).
• Four seasons : spring, summer, autumn, winter.
• Four parts of a day : night, morning, afternoon, evening.
• Four cardinal directions : north, south, east, west.
• ancient capitals of China
• Four corner method
• cardinal principles
• horsemen of the Apocalypse : war, famine, plague, death.
• Four suits of playing cards : hearts, diamonds, clubs, spades.
• The number of tines of most forks.
• Four nations of the United Kingdom: England, Wales, Scotland, Northern Ireland.
• Noble Truths in the Buddhist faith.
• Four estates : politics, administration, judiciary, journalism. Especially in the expression "Fourth Estate", which means journalism.
• There are 4 theme parks that make up the Walt Disney World resort in Orlando, Florida: the Magic Kingdom, Epcot Center, Disney-MGM Studios, and Disney's Animal Kingdom.
• Four Corners is the only location in the United States where visitors can be in four separate states at one time: Colorado, Utah, New Mexico, and Arizona.
• Four conferences whose members contend for the national championship in Division I-A football: SEC, Big Televen, ACC, and either Pac-10 or Big 12.
• There were four Superman movies staring Christopher Reeve.
• Typically there are four movements in a classical symphony or string quartet.

Years

• The years: A.D. 4, 4 B.C..ca:Quatre