It doesnt really matter, just within those times i would give him a bottle. If he looks like hes starting to stir around the 3 hour mark, then get him up and give him a bottle. Otherwise id just let him sleep to the 4 hour mark, but not really go past that. If you do then no big deal, just feed him when you notice the time. He's putting on weight isnt he? Thats all thats important. But he will let you know if hes hungry. You might find if he does sleep for a long period he'll want another feed not long after that inital one he gets when he wakes.