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# Four

Four (4) is the natural number following three and preceding five.

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.

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

Four is also:

Other usages include: