Hugs Mariesusu 4mo is tough! Yes it's probably all those things, and some are even teething too! I do think that this is the age where we have to start being more consistent with things like PUPD, ie if you've started doing it then don't give up until they're hungry or it's been 45mins or whatever your goal is for that particular sleep period. If you give in and use a prop then they can learn to expect that, rather than to learn to go to sleep by themselves. However do your best to work out whether there's anything else bothering them that you can help them with, as PUPD won't work if they're in pain. Also you need to consider what you're willing/able to do right now - if you want to do PUPD then you need to make sure you have lots of support to stick at it. But if you really can't do that right now then do what you have to do to get through this time, but know that you will need to break habits later. But what I've done with both of mine is use the sling for a subsequent nap to try and reduce OT accumulating through the day. I think that's less confusing than starting PUPD in the cot and then switching to a prop, so they know what's happening right from the start of a nap, iyswim?
One of the things we've found with the 4mo regression is that A times can suddenly increase - I tend to think that as a general rule up to 3mo difficulties with falling asleep tend to be OT, whereas UT can be a culprit as they get older, although of course it can also be OT especially as this will tend to accumulate during the day. Just today I know I made the mistake of PD too early and really regretted it - lots of complaining, needed lots of help to calm down etc - oops!
Mariesusu - just wondering about your comment re trying to hold off a feed after the 3rd nap? Do you think he's hungry at that time? We're actually still doing 5 daytime feeds but the last two are getting closer together, so it's more as case of feed, BT routine then top off just before PD (but not feeding to sleep). I only offer that as a suggestion for squeezing in another daytime feed if you think he's extra hungry due to a growth spurt. Actually if he only has an hour A time to BT after a 1.5h nap, that seems quite short to me, you may want to try capping that nap if the others were good, bearing in mind that the 'classic' 4h EASY has the 3rd nap as a catnap. Alternatively stick with the longer nap if that's what he naturally does but give him a little more A time to BT, that would also give you more time to fit in two feeds before BT if you think he would benefit from that during his GS.
CVbaby - I'm so sorry I never got back to you after having to attend to my own 4mo! How's it been going since your original post? It would be really helpful to see your typical EASY to know what specific things to suggest. I would continue to pd awake as you're doing, unfortunately sleep training isn't a one off thing, you will need to keep doing it to some extent whenever there are regressions etc. Sleep does apparently change permanently around 4mo but once they do 'get' IS it shouldn't be as hard to retrain when you need to, I wouldn't even call it retraining really, just using the tools you already have consistently.
Btw you could both come and join us in the 3-6mo birth club (in activity - EASY) if you like, we're quite chatty and all going through (or have just been through) it...