In ancient texts the character 亖 (four horizontal strokes) was used. The current form 四 depicts breathing through the nose and is the original form of 呬. The use of 四 as a number is a phonetic loan.
Example sentences
四月一日です。 — It's April first.
学校は四月からです。 — School begins in April.
一日は二十四時間ある。 — A day has twenty-four hours.
二かける七は十四です。 — Two times seven is fourteen.
これは二千四百六十八ドルです。 — This is two thousand four hundred and sixty-eight dollars.