well trust me i am no expert but u could try to resettle, if re wakes within the hour then i would guess hunger!
is he taking more feeds in the day too? if so could be a growth spurt! if not try to increase his calorie intake in the day and see if that helps!
another thought when did u change to 4 hr routine? could be that he wasnt ready maybe try 3.5hr easy
u could just ignore me though as i have tried everything and my ds is 5.5mths and atill waking
i am sure the mods will have some good advice for u.
Emma