Hi and welcome to the boards!
First off, great choice for hotel and length of time staying - as jill said, you'll have loads of time to do everything.
In my opinion, must dos for your son are:
Buzz, of course!
Big Thunder Mountain
Peter Pan's Flight
Pirates of the Carribean
Art of Animation
Star Tours
Honey I Shrunk the Audience
Also, I'd definitely recommend Cinemagique, your son may not 'get it' all, but he'll enoy the bits he does, but you and your husband will enjoy it all

Don't want to give away any spoilers!
If your son likes the characters (some kids get a bit nervous with them), i'd recommend Cafe Mickey, as it plays disney videos and theres tonnes of character interaction, and the food is fab.
My personal top time-saving tip - don't queue! My mum and I have learned after many trips that what is busy during the day isn't busy in the evening, and fast passes are a god send. You'll just get frustrated, and there's so much to see and do in 4 days you'll get everything done!