In texts and very casual emails, people write 4 for for, because for sounds like the number four. You might see “this is 4 you” or “thx 4 the help”. It is a shortcut, not a new meaning of the number. Though actually, the same digit still means four when someone is counting, so you only read it as for when the sentence wants a little word, not a quantity. Same family of chat spelling as 2 for to/too. Never use it in school work or careful mail.

聊天、簡訊裡用 4 來寫 for（諧音 four）。例如 4 u＝for you。作文不要用。