The earliest oracle-bone scribes counted one, two and three the same way: stack that many horizontal lines. Four lines, though, were too easy to miscount at a glance — so scribes borrowed an unrelated character that happened to sound the same, a small picture of breath escaping from a mouth. That borrowed shape, a box with two strokes branching inside, is the 四 still written today.
A piece of culture
The number nobody wants 四 (sì) sounds almost exactly like 死 (sǐ), "death" — so across much of the Chinese-speaking world, four is deeply unlucky. Buildings skip the 4th floor the way Western ones skip the 13th, and license plates or phone numbers with 4s sell for less. Numbers ending in 8 (发, prosperity) sell for a premium instead — numerology you can hear in the sound of the words themselves.