The 4 (number) reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

4 (number)

Four redirects here; for alternate uses, see Four (disambiguation).

4 (four) is the natural number following 3 and preceding 5.

List of numbers — Integers << 0 1 2 3 4 5 6 7 8 9 >>
Cardinal	Four
Ordinal	Fourth
Numeral system	quartal
Factorization
Roman numeral	IV or IIII
Binary	100
Hexadecimal	4

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.

In addition, . Continuing the pattern in Knuth's up-arrow notation, , 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.

A flat two-dimensional map can always be colored using only four different colors so that all adjacent regions are colored with a different color; three colors are not sufficient. This is the four color theorem.

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.

The number of n-Queens Problem solutions for n = 6.

In other fields

Four (四 pinyin si4) in considered an unlucky number in Chinese culture because it sounds like the word "death" (死 pinyin si3).

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

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
• Four Non Blondes
• Four great ancient capitals of China
• Four corner method
• Four-character idiom
• Four cardinal principles
• Four Books

Four is:

• The number of nitrogenous base types in DNA and RNA.
• The number of strokes in the cycle of a four-stroke engine.
• The atomic number of beryllium.
• the figurative number of corners that the world has.
• the number of musicians in a quartet.
• The number of strings on a violin, a viola, a cello and a double bass.
• 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.

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