Four was originally just four tally lines (亖), but stacked next to 三 it became impossible to read at a glance. So Chinese borrowed a different character — 四, picturing a mouth or nostrils breathing — purely for its sound. A rare case where the picture has nothing to do with the meaning.
四 is China's unlucky number: it sounds almost identical to 死 (sǐ, 'death'). Many buildings have no 4th floor, no room numbers with 4, and gifts in fours are avoided. The fear even has a name — 'tetraphobia'. Knowing this saves you real awkwardness in China.
An enclosure with two legs inside — borrowed sound-alike for FOUR.
四 sounds like 死 (death), so it's deeply unlucky — many Chinese elevators skip the 4th, 14th, and 24th floors entirely, just like Western buildings skip 13.