Hey Lisa,
My son stopped napping at Age 4. However, lots of my friends' kids who were 4 were still napping. They would nap when they were not in school (alternate kindergarten days) and/or the weekends! I wish my son would nap when he wasn't in school :P

If your son wakes up early, then perhaps he still needs a nap during the day. I'm sure he will be out of his nap soon. In the mean time, enjoy the quiet time :)