My first ever trip to WDW, when dd was 5 (turned 6 on that trip), we stayed for four days. No, we didn't see nearly everything, but we got a lot done. And that was before the days of FP and all this planning!!! Imagine waiting in line for three hours, for Splash Mt on a Sat afternoon...yes, we did that because we didn't know any better!!
So, depending on your flight times, coming and going, I might be tempted to add a fifth day to the tickets (not very expensive) and make that last day a second trip to a favorite park. In any case, I would go to DHS. There is a ton of stuff to do there....Toy Story Mania, Star Tours, Jedi Training Academy, Launch Bay, Little Mermaid attraction, Disney Jr, Great Movie Ride, Indiana Jones, and of course Rock n Roller Coaster, and Tower of Terror. Yes, there is a lot of construction going on, but I still find stuff to do.
But, it's up to each family to decide what works best for them. I would sit down and have a family conversation. Ask each person what they really want to do in each park. Birnbaum has a wonderful guide book written just for kids...it gives a great overview of the parks and what's in each one. Then, decide on what park for what day you want. Then you can make FP+ choices depending on what everyone wants to do. That way, everyone gets a say and you have some kind of schedule. Believe me, WDW is no place for spontaneity!!! YOu have to have a plan nowadays!!!