Hi L.,
My son, now 15 months, does this too. He usually goes to sleep around 6:30 or 7 pm and wakes around 5 or 5:30 (on really good days he sleeps till 6). But more than occasionally, he wakes at 4 or 4:30 am. I have tried to get him to go back to sleep or let him cry for a little while and fall back asleep, but it really doesn't work. He will continue to cry, and by "cry" I mean scream, until I go get him and give him his morning bottle (I know, another issue that he is 15 months old and still having a morning bottle). So if it is 4 or later, I usually just get up for the day because I would be up anyway listening to him scream. I'm not saying it's the right thing to do, I'm just telling you what I do. I would love to get that extra hour of sleep too, but sometimes it's just not in the cards.
Good luck!
L.