4 (number)
From Academic Kids

Template:Numbers (digits)  
Cardinal  4 four 
Ordinal  4th fourth 
Numeral system  quaternary 
Factorization  <math> 2^2 <math> 
Divisors  1, 2, 4 
Roman numeral  IV or IIII 
Unicode representation of Roman numeral  Ⅳ, ⅳ 
prefixes  tetra (from Greek) 
Binary  100 
Octal  4 
Duodecimal  4 
Hexadecimal  4 
4 (four) is a number, numeral, and glyph. It is the natural number following 3 and preceding 5.
Contents 
Evolution of the glyph
Missing image
Evolution4glyph.png
Image:Evolution4glyph.png
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, Missing image
TextFigs148.png
Image:TextFigs148.png
.
In mathematics
Four is the smallest composite number, its proper divisors being Template:Num and Template:Num. Four is also a highly composite number. The next highly composite number is Template:Num. 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, <math> 2 + 2 = 2 \times 2 = 2^2 = 4 <math>. Continuing the pattern in Knuth's uparrow notation, <math> 2 \uparrow\uparrow 2 = 2 \uparrow\uparrow\uparrow 2 = 4<math>, and so on, for any number of up arrows.
A foursided 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 noncyclic group has four elements; it is the Klein fourgroup. Four is also the order of the smallest nontrivial groups that are not simple.
The fourcolor theorem states that a planar graph (or, equivalently, a flat map of twodimensional 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 foursquare 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 nQueens 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 3simplex, has four triangular faces and four vertices.
Four is the first positive nonFibonacci 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 allHarshad number and a semimeandric 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 fourdimensional — time and threedimensional 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  ringfinger) 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 (http://www.ngcic.org/) NGC 4, a faint galaxy in the constellation Pisces
The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1175.html) 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 (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1175.html) 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 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 00100_{2} 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.
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".
In the sport of cricket, a four is a specific type of scoring event, scoring four runs.
In Astrology, Cancer is the 4th astrological sign of the Zodiac.
Four is also:
 The number of Greek classical elements (fire, air, water, and earth)
 The only number that, when spelled out in English, has the number of letters that it names.
 The number of seasons: winter, spring, summer and autumn.
 The parts of a day: night, morning, afternoon, evening.
 The number of "corners" of the Earth: North, South, West, East.
 The number of ancient capitals of China
 Four corner method
 The number of characters in a canonical fourcharacter idiom
 The number of cardinal principles
 The number of horsemen of the Apocalypse
 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 tines 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
 In rugby union, The number of the lock forward, who usually jumps at number 2 in the lineout.
 In baseball, four represents the second baseman's position.
da:4 (tal) de:Vier es:Cuatro eo:Kvar fr:4 (nombre) ko:4 ia:4 it:Quattro he:4 (מספר) la:4 nah:Nahui nl:Vier ja:4 no:4 (tall) nn:Talet 4 pt:Quatro sl:4 (število) sv:4 (tal) th:4 (จำนวน) zh:4