Hi! I am taking my son, who will be just shy of 4 to DW in March. We will also be traveling with our 8 mo old, though, so I am trying to compile a list of 4-5 things per park that are "must sees" for our older son- feel free to inlcude dining locations as well! thx!!!!
Mom of 4yo DS here!
MK- Dumbo, Philharmagic, Pooh, Speedway, train, TTA
(Yeah, I know

but DS loves TTA because it goes thru the dark tunnels)
Pinocchio's with a window seat by IASW- they have pizza
Avoid Stitch- too scary! Avoid Laugh Floor- preschoolers sense of humor isn't quite there yet.
Epcot-Nemo/the Seas, Soarin, Living w/Land, Test Track(may be too scary for some), Imagination w/Figment, Spaceship Earth
Love Sunshine Seasons in the Land pavillion for CS.
Avoid Honey I Shrunk the Audience- may be too scary!
DHS-Playhouse Disney, Character greetings(they have Buzz, Cars, Power Rangers), Honey I Shrunk the Kids playground, LMA show or Indiana Jones if he is into cars or Indy like mine, Muppets 3-D.
If he's into Little Einsteins they have a character meal.
AK-Finding Nemo, Kilimanjaro Safaris, either of the animal trails, Dino dig site,
Rafiki's Planet Watch, FOTLK
Avoid ITTBAB- very scary and dark- lots of crying kids!