No personal experience but I can imagine the clean up would get old pretty quick. What does he say? Can he feel a poo coming? Do you think he's doing it on purpose? Do you think there might be a bowel control issue? How do you (and DS) feel about going back to nappies/pull ups? Can he ask for a pull up/nappy when he feels a poo coming? Is he regular or does he display any obvious signs a poo is coming?

I'm guessing he probably doesn't like poo-y pants every day either...I'd be inclined to take him back to the Dr, or another one if I thought there was a problem. Otherwise, 4 isn't unheard of to still be toilet training.