4:30am - ouch!
Well if he doesn't seem tired during the day then it may be UT, rather than OT, that you're dealing with - having said that, if he did his usual BT and then woke at 4:30, that's a very short night and could have been OT catching up with him due to having had several shorter nights...iyswim? Does he still tend to manage his usual naps (as you posted above) when he's woken so early for the day? My suggested plan for you would be something like:
Up 6am at the earliest, even if he woke earlier (if he wakes later, great, just shift the rest of the day by the same amount)
Nap 1: 9am for 20-25mins (may need to play around a bit with the exact times)
Nap 2: 12.30/12.45 (again, will depend on length of am nap, may need trial and error) for 2h
BT: 4h later
This should eventually get you a longer night and then you can try shifting his day later, I'd tend to move everything in 15min increments, hold for a few days, then shift again. Alternatively, you could try moving the first nap a little later straight away and move everything by the same amount, but I wouldn't personally try that if he woke at 4:30am, for example. Wdyt?