1The number four (4); indicates four units or the fourth in a series. Has two reading systems — Sino-Japanese し and native よん / よ — chosen by context and the following word. Because し is a homophone of 死 ('death'), よん or よ is often preferred for euphemistic reasons (e.g. 四時 'four o'clock' = よじ, 四日 'fourth day' = よっか, 四階 'fourth floor' = よんかい). In legal/financial documents 肆 is sometimes used in place of 四 to prevent forgery.
彼女は四人家族で暮らしている。
She lives in a family of four.
会議は午後四時に始まります。
The meeting starts at four o'clock in the afternoon.
日本には春・夏・秋・冬の四季がある。
Japan has four seasons: spring, summer, autumn, and winter.
りんごを四つください。
Four apples, please.
四日の朝に新幹線で出発します。
I'm leaving on the morning of the fourth by Shinkansen.
病院では「四号室」を避けて部屋番号をつけることが多い。
Hospitals often skip 'Room 4' when numbering rooms, because し sounds the same as 死 ('death').
領収書には「金肆万円」と書かれていた。
The receipt was written 金肆万円 ('40,000 yen'), using the legal form 肆 in place of 四.