My almost 4 year old fell from the bicycle in a park and his nose is all scratched. does he need a tetanus booster? he received 4 dtap shots by 18 months age. does he need a post exposure tetanus shot? he will be turning 4 in a month?

NO: This is not a penetrating wound, so the likelihood of tetanus is so low I wouldn't even mention it. If he's due for his 4 year well check next month, just keep that appointment and get his usual 4 year booster then. If you get one before his 4th birthday, he would have to repeat that dose, so I would wait. Keep area clean, wash with warm water and gentle soap.