I think Fleur found that with the pm catnap, even 2h was too long to BT - is that right, Fleur? So by bringing BT earlier you end up with more sleep overall and not so OT. The UT am nap scenario can be prone to EW if you allow that nap to be too long but if 4h is getting a good first nap then I'd stick with it and just pull back on subsequent A times. Be aware that tiredness will build up throughout the day so as Fleur said, if the combined A times are approaching 10h then you'll need to bring BT forward, perhaps by shortening the 2nd nap, but I can't advise whether to make that UT as no experience with long am, short pm.
Btw if you really want to try short am, long pm (not really advising it unless it will work better for your family or the current way isn't working - but you'll need to stick with it for a while longer to tell that), we've had illness and travel etc recently but seem to be settling on something like:
0620 Up
0920-1020 Nap 1
1315-1515 Nap 2
1830/1845 BT (haven't worked that one out yet!)
I do think you'd need to go shorter than 3h 2nd A time if you tried a 30min morning catnap.