4 (four; /ˈfɔər/) is a number, numeral, and glyph. It is the natural number following 3 and preceding 5. Four is the only cardinal numeral in the English language that has the same number of letters as its number value.

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.

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.

Only one number has an aliquot sum of 4 and that is squared prime 9.

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 only composite number n for which (n − 1)! ≡ 0 (mod n) is false.

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