The numeral four used with the general counter 'つ' — indicates a quantity of 4 (used for counting general items).
りんごを四つください。
Please give me four apples.
箱の中にボールが四つ入っている。
There are four balls in the box.
その問題は四つに分かれている。
That problem is divided into four parts.
Indicates age: four years old (used to say that someone, usually a child, is four).
うちの息子は来月で四つになります。
My son will turn four next month.
小さい子が『四つ！』と自分の年を言った。
The little child said, 'Four!' when telling his age.
あの幼稚園児はまだ四つだ。
That kindergartener is still four years old.
In the old Japanese time system, refers to ten o'clock (approximately 10 PM in modern terms) — a historical time expression.
古文書では『四つ』と記されているが、これは旧時刻で十時を意味する。
The old document records 'yottsu' (四つ), which in the old time system means ten o'clock.
A historically derogatory term used to refer to burakumin (a discriminated group). It is a slur and should not be used in modern, respectful speech.
この語は差別的な意味合いがあるため、現代では使わない方がよい。
Because this word carries discriminatory connotations, it is better not to use it today.
In sumo, an abbreviation of 'yotsumi' — a position where both wrestlers have cross grips on each other's belts (cross grips).
二人は四つになって力を尽くしたが、押し切れなかった。
The two locked into a yotsu (cross-grip) and put in all their effort, but couldn't push the other out.
四つに組んだら腰の使い方が勝敗を分ける。
Once in a yotsu (cross-grip), the use of the hips determines victory or defeat.