our DSs are 3 and 5. Our 1st WDW trip was 2 years ago when they were 1 and 3. We've been back twice since then. Start at MK in Fantasyland as soon as the park opens. Ride Dumbo first because the line for that can be sooooooo long. Other "must sees" are Pooh (fastpass), Peter Pan, Small World. Go to Toontown-go thru Mickey and Minnie's houses, play in Donald's boat and small play area for toddlers next to it. Goofy's barnstormer is a good first roller coaster for your 4 year old. Other MK "must sees", according to our DSs are Buzz Lightyear, TTA, Tomorrowland Speedway, train, Jungle Cruise, Country Bear Jamboree, Paddle/river boat. Of course, DSs' favorite ride is the monorail-lol. At AK, DSs recommend the safari, the petting zoo in Rafiki's area, Pocahontas' show and the Lion KIng show, and the Boneyard play area. At MGM, the must sees are Bear in the Big Blue House and Little Mermaid. It's also the only place to find Buzz Lightyear for pics and autograph. We have taken DSs to Epcot each trip but usually just for part of the day. AK and MGM can also be seen in less than a full day unless you plan to "baby swap" and ride the "e ticket" rides. We spend the most time at MK. Also, definitely make ressies for a character meal. Hope this helps. Pamela