四

四 (sì) is the Chinese number 'four.' It works like any other digit: combine with measure words for counting (四个人 'four people,' 四本书 'four books'), with 月 for the month (四月 'April'), and within larger numbers (十四 'fourteen,' 四十 'forty,' 四百 'four hundred'). The cultural note matters: 四 sounds nearly identical to 死 (sǐ, 'death'), differing only in tone, so the number four carries a strong unlucky association across Chinese-speaking cultures — many buildings skip the 4th, 14th, and 24th floors, hospitals avoid room 4, and phone numbers/license plates with multiple 4s are cheaper. This is the East Asian equivalent of Western tetraphobia (and parallels Western avoidance of 13). For learners, this cultural fact is as important as the number itself.

Memory hook: 四 looks like a box with two legs inside — picture 4 walls and a doorway. The unlucky 'death' echo is a coincidence of pronunciation, not character meaning.