I personally would choose DHS, but that's just me, for a four year old, depending on height and how much thrill they like,and if they can sit through shows, here's what I'd say is at each park:
DHS:
-Toy Story Mania
-Disney Jr Show
-Little Mermaid Show
-Beauty and the Beast show
-Muppets 3D
-Great Movie Ride
-Fantasmic
-Character meet and greets [including the fab five, chipmunks, buzz and woody, stitch, daisy, wreck it ralph, disney jr characters]
AK:
-Safari
-Kali River Rapids
-Triceratops Spin
-Primeval Whirl
-Boneyard playground
-Festival of the Lion King show
-Finding Nemo musical
-Animal viewing trails
-Character meet and greets [including Winnie the pooh and friends, mickey and the gang, Pocahontas, mickey and the gang, king louie and baloo, Russell and dug]
If they like animals more, and can spend hours at a zoo, then I'd say AK, but if they prefer to see more, I'd say DHS, there just seems to be more to do there, especially if they have the ability to sit through shows.