We were just at Universal for the first time, and although my kids are a bit older, my four-year-old LOVED LOVED LOVED LOVED LOVED Seuss Landing at Islands of Adventure. If your child knows the Dr. Seuss books, then that is the place to go. You could easily spend a morning there. There is a playground (if I ran the zoo), rides (Cat in the Hat and One Fish Two Fish) and a carouseussel (Carousel) which is great. It is also bright and visually appealing for kids and just magical. We really enjoyed it. There are also places to eat my son really enjoyed the Green Eggs and Ham. He also enjoyed the Jurassic Park play area and the discovery center and the Olive ship at Toon Lagoon.
At Universal he liked the Curious George play area and could have stayed there a long time, but there seems to be less for little ones. Shrek is intense and possibly scary with a visit to a cemetery etc so keep that in mind. We loved it, but my DS, 4 would not go on it twice. He did like ET a lot.
I would suggest IOA instead of UO if you only have one day with a child that age.