I have a list of must do's for my upcoming Grand Gathering Trip. We have a lot of personalities and ages but here is what I am recommending they do not miss - some will work for 4y/o some will not.
Magic Kingdom - Splash Mountain, Pirates of the Caribbean, Crystal Palace, Nighttime parade and fireworks, Mickey's Philharmagic, Buzz Lightyear, winnie-the-pooh, Haunted Mansion, Big Thunder Mountain Railroad, and Dumbo.
Epcot - Soarin, Turtle Talk with Crush (also new nemo ride), Test Track, World Showcase and the fireworks
Animal Kingdom- Dinosaur, Expedition Everest, It's tough to be a bug, Lion King, Kilimanjaro Safari, afternoon parade. I will probably add the nemo musical.
Disney MGM studios - FANTASMIC, MUPPET VISION, little mermaid, beauty and the beast, tower of terror.
Also check WDW website for toddler and preschool friendly activities.
Hope this helps.