but other times really fights the naps so I guess at that point he's OT or UT.
Yes probably. It can be linked to the length of time he has been awake:
https://babywhispererforums.com/index.php?topic=84884.0and can be linked to the level of stimulation during that A time (ie higher than usual he may be tired earlier or OT/OS, but if it was less stimulating he might be UT/US)
or it can be linked to sleep training which is a gradual learning process.
It's quite normal for there to be easier and harder naps though, so please don't feel it is anything you are doing wrong or anything you can necessarily "fix" easily. Just keep recording your times and activities, looking for patterns, and gradually you will get to learn the wind down length and timing a little better.
The early waking might be due to OT, if the naps were short the day before then you may need to fit in another nap or shorten the last A time before BT to try to avoid OT. Sometimes when parents try to follow the routine in the book closely but get short naps they end up with a very long A time left over at the end of the day, that can be the cause of OT.
I wouldn't really put down for bed earlier after a 4am start as it could perpetuate the early start, probably better to fit in another nap even if you have to APOP it (accidental parenting on purpose, ie do whatever it takes to get him to sleep, in arms is fine), an extra nap should help to reach a "normal" BT or closer to it.
hope this helps