Yep. I had the same issue throughout my pregnancy (seemed almost from the moment of the 2 lines on the stick!)
Early morning waking, if you don't have to pee, is often due to stress or "physical depression" (you don't have to feel sad), hormones, and changing serotonin levels. Now, with all the changes in your body, 3 other children, one on the way, and a full time job, if it was me...I would be surprised if I could sleep at all!
Do you "think" a lot when you wake early? If you have the "nonstop mind" problem, have a journal and flashlight by the bed, and write, write, write. Get everything out of your head onto paper (even if it makes no sense)..."to-do" lists, concerns, feelings, whatever. It really can work!
If you are taking prenatal vits, see if you can take them in the morning (if you are taking them at night) as long as they don't make you nauseaus. The B vitamins in there can affect your sleep and make you a little energized. If you can drink milk, have a glass of milk before bed or take a calcium/magnesium supplement...it's safe and can help your sleep cycle. Try to do some stretching before bed, or if you do yoga, a few relaxing poses. A warm shower and then a cool bedroom can also help. All of these things are ways to trigger your body to "adjust" the sleep hormones naturally.
Turn your clock around so you can't see the time (and just set your alarm). Clock-watching can actually cause insomnia in the early morning hours.
Hang in there and best wishes to you and your wonderful family!