We did Universal in California last year, so a little different than the Orlando one, but we had DD2 along, and she was 4 at the time, although shorter than your 4 year old.
In HP world, she was able to ride Flight of the Hippogriff and loved it. We did rider swap on the Hogwarts ride, which involved her sitting in a room off to the side with whichever parent wasn't riding & watching the first HP movie.
She loved Shrek & Simpsons and meeting the characters. And she was tall enough for the Transformers ride. Oh, and Despicable Me. She couldn't ride Jurassic Park, so we stayed in the rider swap area there, as well, and played with the toys they had.
Honestly, I was worried about taking her, but we ended up having more to do than I would have thought. And Florida seems to have even more for the smaller guests, with Suess Landing, etc.
-Kris
DS (9/05)
DD (8/08)
DD (9/12)