四 is considered unlucky because it sounds like 死 (death). Many buildings in China skip the 4th floor (and 14th, 24th, etc.), similar to how some Western buildings skip the 13th floor. Phone numbers and addresses with 四 are avoided.
culture
Despite the superstition, 四 appears in many positive traditional phrases: 四面八方 (all directions), 四季如春 (four seasons like spring, meaning pleasant year-round).
Outer enclosure radical - the indexing component. The big square frame (not the small mouth 口). Originally 四 was four horizontal strokes; the modern shape was borrowed from a pictograph of nostrils or a mouth open to breathe. The frame strokes are written first and last, with the inner content slotted between.
Inside, two short legs/strokes splaying outward - graphically what was once a tongue-like or breath-like inner mark. Has no etymological number-meaning here; treat the inner pair as a memory hook (two legs inside a frame = the digit four, written second and third).
