R.S.
Wait. He may be educationally ready, but if his physical skills are at a 4-year-old level while all his classmates are older and better-coordinated, if he's always the last picked for teams, if he can't do what most others in his class can do, what will that do to him? Better to wait until he is really ready. I don't have a December boy, but I have a December birthday, and my parents were faced with the same decision. They waited until I was 5. It has never mattered. And I wouldn't call waiting until he is 5 "holding back". Kids just aren't ready at 4. My son is almost 4, and there is NO WAY he'd be ready.