I'd say hunger too. Not 'major' hunger that makes a baby shout for attention. But a grumbly enough stomach that it makes it hard for him to go back to sleep.
I assume he has breakfast when he gets up? An hour and a half earlier won't make much difference in terms of hunger - he's likely to be approximately as hungry at 4.30am as he is at 6.30 when he has breakfast.
It's quite telling that not long after eating breakfast he has a nap.
Does he take milk feeds OK at other times? It is not unusual for some babies to start refusing milk feeds once weaned. But in fact they do still need milk feeds so you have to get creating in finding ways to encourage drinking milk.
I would try introducing a dreamfeed at 11pm (or whenever you go to bed) because I find a half-asleep feed the easiest time to get milk into an older baby. This feed 5 hours after bedtime should help the hunger pains through to morning.
Also bear in mind that changes take a while to be established. You'll need to give it a couple of weeks to be established.
Alternatively you could feed quickly when just woke up and still dozy in the morning at 4.30am, instead of the dreamfeed.
Once you break the early wake up cycle, it becomes self perpetuating. When hes waking later then all his naps move later, which means bedtime can be later, which reaffirms the later mornings. It's just about breaking that cycle first.