Linocut, unique state, hand coloured (watercolour)
4 (four) is a number, numeral, and glyph. It is the natural number following 3 and preceding 5. Four is the only number in the English language for which the number of letters in its name is equal to the number itself. This is also true in several other languages, such as German, Afrikaans, Dutch, and Flemish, as “vier”.
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 6.
Four is the second square number, the second centered triangular number.
4 is the smallest squared prime (p2) and the only even number in this form. It has an aliquot sum of 3 which is itself prime. The aliquot sequence of 4 has 4 members (4,3,1,0) and is accordingly the first member of the 3-aliquot tree.
Only one number has an aliquot sum of 4 and that is squared prime 9
The prime factorization of four is two times two.
Four is the smallest composite number that is equal to the sum of its prime factors. (As a consequence of this, it is the smallest Smith number). However, it is the largest (and only) composite number n for which is false.