Here's what I would do:
MK: Extra-Magic Hour, hit Peter Pan, Snow White, and Mad Tea Party. When the rest of the park opens, do Big Thunder Mountain, Cinderella's Surprise Celebration, Buzz Lightyear, then head to Town Square and the Expo Hall for character meet-and-greets.
EPCOT: Afternoon, Do as much of World Showcase as possible. GO through all the countries seeing the entertainment and hit Maelstrom, Impressions de France, and El Rio Del Tiempo if there's time. Have lunch in one of the countries
MGM: Afternoon. I love ths shows, but with only 4 hours in the parks, there's really not enough time to see them and anything else, so I would stick with the rides. Start with Tower of Terror, Animation Tour, Great Movie Ride, and end with the Parade.
