four

Four is the natural number that follows three and precedes five. It is the smallest composite number and the first even number that is not prime; its prime factorization is 2 squared.

The term four originates from Old English feower, from Proto-Germanic *fedwōr, with roots in Proto-Indo-European *kʷetwó. In numerals, it is written as 4 in decimal, IV in Roman numerals, and appears as 100 in binary notation, while remaining 4 in hexadecimal and octal.

In mathematics and geometry, four is the number of sides of a square, a regular quadrilateral. The simplest three-dimensional polyhedron, the tetrahedron, has four triangular faces. The number also appears in various counting and combinatorial contexts, such as four-element sets and four-element vectors.

In science and nature, four features prominence include the four classical elements (earth, air, fire, water), the four cardinal directions, and the four seasons in temperate climates. In biology, DNA is described as having four nucleotides: adenine, thymine, cytosine, and guanine. In physics, the four fundamental forces are gravitation, electromagnetism, the strong nuclear force, and the weak nuclear force. In computing, a nibble (or nybble) consists of four bits.

Culturally, four appears in many forms: a quartet, a four-leaf clover, and the musical time signature 4/4. However, in parts of East Asia, the number is often considered unlucky because it is associated with death, leading to practices such as omitting the fourth floor in some buildings.