Heya!
Well, as I'm sure you know, moving to a 4 hour EASY is not exactly...umm...easy. It's during this time that your lo goes through the biological change where day sleep develops, and tbh, the best advice is to just keep your chin up and whether through it, go with your instinct, because much of the time, no matter what you do, you're going to have 45 min naps, with a few long ones in there occasionally. You know you need to push toward a 2 hour wake time, but He probably won't be up to 2 hours solid until he's closer to 5 months, if that's any help.
At this point you can start watching the clock more than sleepy signs, because they will be confusing. It was at this point I started pulling my own hair out with Aari because she was doing exactly what your lo is doing. That line between overtired and undertired is pretty shaky at best, I swear until day sleep development is over it changes daily!
After it's over though, it's all about not being up long enough. He'll start taking normal naps again (hopefully lol), and if he regresses back to 45 min naps, you know it's time to extend "A" time again.
Sorry I can't give you some hard and fast answers...every lo goes through this a little differently, but based on what other moms who have gone through this posted, and based on what my lo did, this is the best advice I can offer!
*HUGS*
Sonya =P