Kristen, a few more thoughts. Please disregard if I don't make any sense.
Okay, firstly, I think that there is just so much accumulated OT that you can't really go by the A times as they are all over the place. And you are getting different nap lengths even with similar times - eg: One day after 3.5, you have got a 2 hour nap but then that hasn't repeated. I agree with what you said -- I think for a few days, if possible of course -- just let him nap for as long as he needs for the first nap and then you can reassess the day based on that. If nothing else, he can catch up on some OT.
Another thing I found with E, if I go in too early, I get massive resistance and then she gets super OT by then end and crashes, and I inevitably get a short nap and a cranky baby. I am mainly speaking about this --> Ended up with 4 hour A after long nap, but tried to get him down starting at the 3.15 mark. So his A time isn't 4, it just took so very long because once they get into that mood, it is hard to snap them out of it. Whereas here you did not have a problem ---> Took DS into room at 3.75 mark fell asleep easily at 4 hour mark.
I think L needs a short wind down otherwise like E, once he starts resisting, it becomes hard to get him down and more OT buildup
I like your idea of just going with the flow and letting him sleep in the day (for a day or two, at least) and pushing BT a little to adjust. It will at least help getting ahead of the OT a bit and then you can better assess his A time.
FWIW, L & E were doing similar times previously and right now I believe her A is 3.50, so I don't think you are wrong in thinking his is 4 hrs. But as Katherine said, the A before bed shouldn't be too long otherwise the OT monster is back.
Is he still waking every hour in the night? Could you post what the last few nights have looked like?