4 explained.

4 in everyday use

Four is the atomic number of beryllium and the number of seasons, cardinal directions, and suits in a deck of cards. Quarters divide the year, the hour, the football game, and the dollar. In Mandarin, Cantonese, Japanese, and Korean the word for four sounds close to the word for death, so many buildings in East Asia skip a fourth floor much as Western buildings skip a thirteenth. The four-leaf clover owes its reputation to rarity: roughly one clover in five thousand has an extra leaf.

Four is the smallest composite number and the first square above one, being 2 × 2. Lagrange proved in 1770 that every whole number is the sum of at most four squares, so 7 needs all four (4 + 1 + 1 + 1) but nothing ever needs five. The four-colour theorem, settled by computer in 1976, says any flat map can be coloured with four colours so no two neighbouring regions match. A number divides by 4 whenever its last two digits do.