Hi hun, 4mo is a rubbish time for sleep as there's a massive sleep regression. Just look at all the previous posts on these boards about 4mo LO's!
1hr45-2hrs A is around average for this age. It may be that a tiny increase in the A time might help him transition a little easier, but unfortunately some lo's will need help during this SR time. What to try a 10mims increase and see if that helps? It might help shorten the last A a little if there's an extra 10mins before each nap? Wdyt?