Hello welcome to BW forums
If your DS is adaptable to the different routines on different days (he does seem to be based on your two different routines) then I'd suggest a home routine where the CN is dropped - and if possible move the two naps a bit later so that it is not a very long A time to BT or do a slightly earlier BT. And a not-home routine where you keep the 4.30pm CN - and if possible drop one of the morning CNs instead so that the day becomes CN, long nap, CN. With one of the morning CNs dropped he might be more willing to go down for the late afternoon CN to make that easier but each A time wouldn't get overly long. It's just that if you reduce the late afternoon one and then drop it there is a very long A time between nap and BT and I assume you have the school run to fit in.
Many people have non-standard routines to fit around school etc, if you find something that suits you and your LO and the family then that's ideal even if it isn't two long naps every day.
hope this helps