Under 4. I would probably say Universal Studios, but IOA has some great things as well.
There is Barney at Universal, but also Suess Land at IOA. (The Cat in the Hat ride is great!) The Jurassic park discovery center is cool for the younger set as well.
There is also wonderful cartoon characters, ME ship the Olive, Popeye and all that at IOA.
Universal has ET, and Woody Woodpecker coaster, as well as the Hannah Barbara ride.
Really, what is the rest of the party like? Do they like coasters? or more "sedate" rides? Are they big thrill seekers? In IOA, The older kids willl love Spider Man, Hulk and the Jurassic Park ride. If you have a tallish 4 year old they can enjoy those rides as well. (I think the requirement is 42 inches) Storm Force is a cool teacup type ride for all ages
I went last year and enjoyed both parks. WE are visiting again in August, and will take our 4 and 7 year old to both parks.
My 7 year old is more interested in IOA if that means anything.
Feel free to check out the Universal Board for much more info.