My 4 yo fell backwards off kitchen chair and hit back of his head on the tile. cried for a bit but he seems ok now. go to er? wake him up at night?

Head injury: Use ice packs to the back of his head to limit swelling. If he cried immediately and now is fine, you can just observe at home. If he were to start vomiting, not acting fine (nonsense speech, balance issues, daytime sleepiness with difficult arousal etc) then get him evaluated immediately. If he goes all day without issue then waking him at night is not necessary.