If possible, I would buy the 2 park pass too, but if I had to choose one I guess it would be Universal for the kids. Its has woody's coaster, Jimmy Nuetron, Fievels playland, Barney, etc.
BUT, if your four year old makes the height requirements and likes more thrilling rides, then at IOA you have Spiderman, Jurassic park, Suess landing, Popeye, etc, but I don't know if a four year old would make the height restictions.