Park Hop! Are you staying on-site? If so this is what we do...get out early in the am (less heat, less crowded), enjoy a park for a few hours. Either eat lunch in the park and then head back to the hotel, or head back to the hotel, eat lunch and then take a break (nap and/or swim), then head back out around 4:00 - 4:30 to another park (or the same one as the am).
This has worked perfectly on all of our trips, and we've been travelling with little ones of all ages.
Trip 1 - DD 1 1/2
Trip 2 - DD 2 1/2, DD 8 months
Trip 3 - DD 4, DD 2, DS 10 months
Trip 4 - DD 6, DD 4, DS almost 3
Trip 5 - DD 7, DD 5, DS almost 4
I wouldn't do it any other way. We love park hopping -- and taking breaks!
Four days gives you 8 chunks of time (8 half days) for the parks. I'd do 1 half day at AK, 1 half day at Epcot (evening), 1 half day at MGM, and 4 at MK (giving you 2 days for MK), and then you'd still have 1 half day left to decide where you want to spend more time.
Hope that helps. Enjoy!