# PNG images: 4

4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest composite number, and is considered unlucky in many East Asian cultures.

Four is the smallest composite number, its proper divisors being 1 and 2.

4 is the smallest squared prime (p2) and the only even number in this form. 4 is also the only square one more than a prime number.

A number is a multiple of 4 if its last two digits are a multiple of 4. For example, 1092 is a multiple of 4 because 92 = 4 × 23.

In addition, 2 + 2 = 2 × 2 = 22 = 4. Continuing the pattern in Knuth's up-arrow notation, 2 ↑↑ 2 = 2 ↑↑↑ 2 = 4, and so on, for any number of up arrows. (That is, 2 [n] 2 = 4 for every positive integer n, where a [n] b is the hyperoperation.)

A four-sided plane figure is a quadrilateral (quadrangle), sometimes also called a tetragon. It can be further classified as a rectangle, oblong, square, kite, or rhombus.

A solid figure with four faces as well as four vertices is a tetrahedron, and 4 is the smallest possible number of faces (as well as vertices) of a polyhedron. 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. It is the only self-dual regular polyhedron.