My 4 year old LOVED World Showcase, but didn't really like Epcot's Future World except for Figment. In WS, he loved the Mexico and Norway rides, the train village at Germany with the bridge to run around, the hedge maze in UK, and ALL the Kidcot stations. He also enjoyed the living statues and all the characters that we saw there.
Most of the must-do's in the other parks have been mentioned. He actually liked Tower of Terror, though he said "Once was enough." And he's kind of a daredevil. At MGM, he liked the Disney Playhouse and Beauty and the Beast shows, Pizza Planet arcade and statues, Muppet Vision, and seeing characters at Al's Toy Barn.
At MK, I would add to the list Toontown's play area, roller coaster, and Donald's Boat. We also spent lots of time on Tom Sawyer Island. It's a nice place to go when it's hot. He liked the Jungle Cruise and Pirates of the Carribean, but did not care for Haunted Mansion on this trip. He also enjoyed Mickey's Philarmagic.
The only thing he liked at AK was digging for fossils in the Boneyard

The Lion King show was okay for him, but he did not enjoy it as much as the MGM shows.
He LOVED the Crystal Palace character breakfast, but did not really care for the Donald's Breakfastasaurus. The food is also much better at Crystal Palace, and I think it may be a little bit cheaper than Donald's.