It looks like you've gotten a lot of great advice. I have a daughter that is two, and Hollywood Studios has been great in our experience with her. There isn't a ton she can do in terms of rides, but the Disney Junior stuff is great for her along with all of the other things there that have already been mentioned. Other posters are definitely correct that character lines are much shorter at Hollywood Studios and the Frozen area is great!
You said you were going 4 days. Does that mean you will be doing a day at Animal Kingdom? Our daughter loves animals and the safari ride, but we have a great zoo at home that puts her closer to the animals and there aren't many ride options for her at that park either. If you were planning on doing Animal Kingdom, I would switch that day with Hollywood Studios. If not, I would agree and say spend two days at Magic Kingdom and a day at Epcot. Hollywood Studios is a great park, but two days at Magic Kingdom would be time well spent.