J.S.
This is what we'd do with our oldest son. I would tell him that it is nap time, and he needs to sleep. I'd say that if he expects to go out (or whatever other fun thing) then he needs to sleep. We'd tell him if he didn't sleep, then he would stay home (you have to MEAN it). I would also say that he didn't HAVE to sleep, but he had to stay in bed and rest. I'd leave him a book to look at. He had to stay in his room for an hour for rest time.
Four hours is too long to battle him for a nap. If he's not sleeping after an hour, then you need to look at adjusting the time he goes down for the nap. Try putting him down 30 minutes earlier, or tiring him out during the morning. Also, maybe he is just used to the nap routine at daycare. Find out how they do naps and do the same thing. You may need to structure his whole day like they do at the daycare to make things work. If all else fails and he will not nap, then put him to bed earlier. Good luck!