Oh yes, there is plenty for a 4 year old to do. Does he like Barney? There's a show for that (and it's not nearly as annoying as the TV show is ... it's actually kinda cute once or twice). The ET ride (34") can be a little scary, because it's dark to start out before you get to ET's home planet (it brightens up a bit there). If he's freaked out by the forest queue, he'll probably be freaked out by the ride itself. The Fievel/Curious George play areas are a blast, even for parents. There's a Woody Woodpecker kiddie coaster (36") he might be able to ride. The Animal Actors show is great for young ones, nothing scary goes on. If he handled ET OK, and he's tall enough, he might like The Simpsons (40") and MIB (42") rides; Earthquake, too, which doesn't have a height requirement. If not, and you guys want to do them, child swap. The Beetlejuice show is a music show, not scary, he might like that one. Mummy is an adult ride, Twister might be too much for him (it's pretty in your face); Shrek 4-D he might like, same with Despicable Me (he can take off the 3-D glasses if they get too scary). Definitely stay away from Terminator and Horror Makeup shows, and probably Transformers.
For IoA, he'll like everything in Seuss Landing; make sure you don't miss the "If I Ran the Zoo" play area. Poseidon's Fury would probably scare him, but the Sinbad stunt show he might like. Dragons and Forbidden Journey will be a no for him, but he might like the Hippogriff coaster (36"). The Discovery Center in Jurassic Park will be fun for him, and the Camp Jurassic play area is a great place to burn off energy; it has an elevated ride called Pteranodon Flyers (36") he can ride with one of you. Toon Lagoon has Bluto's Bilge Rat Barges (42") that's a fairly typical river rapids ride; Me Ship the Olive is a kid's play area. In Marvel area the only thing he'd probably do/like is Storm Force, unless he's tall enough for Spiderman (40") and likes super heroes.
Not sure when you're planning on going, but if it's soon Mardi Gras will still be happening, and most little kids love it. It's not like the New Orleans Mardi Gras, it's very kid and family friendly. If you're going later in the year, the Harry Potter area at USF will be open, and that will have kid-friendly shops if nothing else.