Early rising is a problem that many older babies and toddlers (and their parents) suffer from. Your toddler is only young at the moment and this may be a phase he is going through that stems from stage of development he is currently going through. At this age he has just learnt that he can keep himself awake if he wants to and he may just be practising this new skill. However, early risers are difficult to deal with. In theory, if you put him to bed later he should sleep in longer, but this measure does not always work in practice. You can end up with a child who still rises early, but is very grumpy because he went to bed late. Another measure I sometimes suggest is filling the cot with soft toys so that the baby can play quietly without waking the rest of the family, but this rarely works, as the baby seems to want to get up and get moving. The controlled crying technique can be useful in this situation, but you can get into the ridiculous situation of settling the baby back off to sleep just when you need to get up. If this looks likely to continue, the much better solution is for the parents to change their sleeping habits to wake with the baby. Not much of a comfort I know, but sometimes it is the only thing that works. Finding out why your toddler is suddenly awake in the night will be a matter of elimination. I presume, that as he sleeps well normally, he can get herself off to sleep, which is very important with sleep problems. How much sleep does he get in the day? It could be that as he is getting older, he now needs less sleep in the daytime. Try not to let him sleep for more than an hour in the afternoon and not after 4.00 pm. Check also that the room is warm enough so that she is not waking up cold and that she is eating enough through the day.
If you have tried all of these and he is still waking early it could be a habit he is getting into. If so, it would be best just to put him back to bed nicely, but firmly. Bringing him into your bed could become a reward for waking up later. Toddlers and young children do tend to wake early, but 4.30am is too early! You may need to keep putting him back to bed until he gets the message.