My Brooke is starting to miss the odd day sleep at the moment, and she's usually pretty reliable with her sleeping.
Hopefully it's the big molars coming through, cos it's better for her and I that she does have a little nap. 
I've heard of 4 yr olds who still have afternoon naps - I don't think mine is going to be one of those. 
Hopefully it is just a phase of 'awareness' your son is going through. I've noticed Brooke now does a bit of babbling before falling asleep - just because she can force herself to stay awake for a bit! It's good though that your son otherwise plays quietly and stays put.