Etymology: Four was once shown by four horizontal strokes.
Interestingly, this character originally meant breath (see breathe emerging from mouth 口).
四 became a phonetic substitute, and also it resembles an image of a closed fist (if close your left eye and tilt your head).
Mnemonic: FOUR FINGERS IN CLENCHED FIST
四月 (shigatsu) April
四日 (yokka) the fourth day
四回 (yonkai) four times
(based upon Henshall, 1998)