This is not what you want to hear... BUT... my two (now 4 1/2 and nearly 3) have always been early risers. About 6am is normal. My older one is not napping anymore, younger one naps maybe an hour, about 3 times a week. He's starting to give that up.
Well, I gotta tell you - my kids have been getting up early almost since day 1. I am not a morning person, but I've learned to deal with it. I say we don't get up until there's a 6 on the clock, but it's usually about 6:02 when she wakes up! I try to have them in bed about 7:30. Unfortunately, in all these years, no matter what time they go to bed, they are up around 6. Changing bedtime has made no difference, even when we might be out at a special occasion and they're up until 9, they still wake at 6. Some kids are just like this. Everyone tells me I'll be glad when they go to school because they'll be up and ready and I won't be trying to drag them out of bed.
Try pushing bedtime forward a little, maybe try to give your 4 yr old some "quiet time" but not necessarily a nap. But don't be too surprised if you only gain maybe a half hour in the am. Adjusting bedtime doesn't work for a lot of kids. They are just morning people. My friend's 2 sons are the same age and they get up early just like mine! Sigh.
*Oh, also - PS - my kids sleep in the DARK. They both have blackout curtains AND black felt over their windows. No night lights. People joke that it's like a cave in their rooms! And they still wake around 6, sunlight or not.