4

4 is the natural number that follows 3 and precedes 5. It is an even integer and the smallest composite number. In decimal notation it is written as 4; in binary it is 100, and in common alternate bases such as octal and hexadecimal it remains 4. In Roman numerals it is written IV, although some clock faces use IIII instead.

Mathematically, 4 has the prime factorization 2^2. Its positive divisors are 1, 2, and 4, so it has three divisors in total. It is a perfect square, being 2 squared, and it appears in the Pythagorean triple 3–4–5 as a leg. The sum of its positive divisors is 7.

In geometry, four is the number of sides of a quadrilateral, including squares and rectangles. In higher mathematics, four appears in various contexts such as the four-dimensional space often discussed in geometry and physics.

In everyday contexts, several familiar concepts involve the number four: the four seasons, the four cardinal directions, and the classical four elements (earth, water, air, fire) in various philosophical traditions. The four basic arithmetic operations (addition, subtraction, multiplication, division) are also grouped around the value.

In science and technology, the atomic number of beryllium is 4. In computing, a nibble or quartet consists of four bits, and some early data representations used four-bit groups before the byte became standard.