This is what I would suggest, but it's totally up to you! Note that this plan does require parkhoppers. If I didn't have parkhoppers and didn't have full days on Day 1 and Day 5 (due to travel), then I would skip the Animal Kingdom altogether and do Epcot as Day 3, MK as Day 4, and "kid's choice" on your departure day.
Day 1 - MK, perhaps arriving late due to travel. Tour Adventureland, Frontierland and Liberty Square. Make FP+ for Splash/BTMRR if you and/or your wife want to do them (4-year-old may not be tall enough or may be too scared, but there's always Child Swap). Use last FP+ for Jungle Cruise or Peter Pan. Dinner ADR at the Crystal Palace or Liberty Tree Tavern. After dinner, start the attractions in Fantasyland (e.g., it's a small world, Mickey's Philharmagic, Peter Pan if you have a FP+ for it).
Day 2 - HS. Arrive at rope drop for TSMM (or Jedi Training Academy, if you want to do that), then (if you/wife interested) RnR/ToT. Then tour park at your leisure. With a 4-year-old, I'd focus on the Disney, Jr! show, Little Mermaid, Star Tours, Muppets 3D, the "Honey I Shrunk the Kids" play area, any meet-and-greets that really interest your son, the Great Movie Ride (cover his eyes during the Alien scene if he might get scared), possibly the Indiana Jones Stunt Spectacular, and TSMM again (use a FP+ the second time around). Take a mid-day break for naps. Don't bother with TS at HS - have dinner at Coronado Springs and spend the rest of the evening by that gorgeous pool, or head to Downtown Disney for dinner.
Day 3 - AK/Epcot. Arrive at rope drop for Kilimanjaro Safaris and (if you/wife riding it) Expedition Everest. Then tour as desired. Tusker House is a good character breakfast here if your son wants to meet Mickey and friends. Return to your hotel for naps after lunch. Then head to Epcot and tour Future World (you'll have to FP+ Soarin' or Test Track, and skip the other one for now). TS dinner at The Coral Reef or CS dinner at Sunshine Seasons.
Day 4 - Epcot/MK. Hit Test Track or Soarin' (whichever one you haven't done yet, or do both!) at rope drop. Finish anything you missed in Future World and tour World Showcase when it opens at 11:00am. Your 4-year-old may get bored quickly in WS, so just hit the highlights. Return to the hotel for a nap. In the afternoon, head to MK and finish Fantasyland and Tomorrowland, making FP+ for headliners (e.g., 7DMT if it's open, Enchanted Tales w/Belle, Space Mountain or Buzz). Stay late for the parade and fireworks. ADR for dinner at Be Our Guest or one of the monorail resort restaurants (e.g., 'Ohana at the Polynesian, Chef Mickey's at the Contemporary).
Day 5 - Revisit your son's desired park. Make advance FP+ in MK, but change them if he chooses something else.
