I came to this thread ready to recommend either BC/YC or Boardwalk because of the pools, the proximity to Boardwalk attractions, the boat to Epcot and MGM (for some reason my sons think the boat ride to the parks is one of the best parts of the trip), and the overall layout and amenities of the resorts. After reading all of the other responses, however, I can't disagree with any of them. AKL, WL and the monorail resorts all have great advantages for a 4 year old. I guess now I am of the view that you can't go wrong with any of them! Your decision should probably be based upon your son's individual likes and dislikes. What will he like best? Seeing the animals at AKL? The overall theming and fun of WL? The convenience and "wow" factor of the monorail? The fun, attractions and proximity to Epcot and MGM of Boardwalk or BC/YC? In the end, your son probably won't appreciate the room accomodations that make these resorts "deluxe," but he will like the extra fun features that each can offer. Any choice will be a good one, but the best choice is the one that appeals to your son's individual preferences.
Also, don't completely overlook your own preferences. He will have fun no matter where you stay, but if you strongly prefer one of the deluxe resorts for your own reasons, please consider yourself as well as your son. Whatever you choose, have a great time!