If the Fantasyland expansion was truly complete I would lean that way but the seven dwarves coaster is still under construction.Since you have never been to IOA I would probably suggest you visit there,however since IOA has more rides geared to older kids there might not be enough for your 4 year old,there is the Seuss landing area,the smaller roller coaster in the Potter area,the playground area in Jurassic Park with the Pterenadon Flyers and a few other things so there are things there along with Potter and all the major attractions like Spider Man,research the park and see if you would rather visit it now or do MK again and wait until your son is a bit older.If you can't decide go with Sea World if you've never been,it's a park everyone loves and has plenty of things for everybody.