Your son can ride many, many rides in the park. I'm assuming most of these would be with you, as I can't imagine putting a 4-year-old on the rides alone. Besides, it's a good excuse for you to not miss out on the fun as well! A few of the rides do have a 40" height requirement, meaning the child cannot ride at all unless they are 40". Even fewer rides have a 44" height requirement. We came to the parks frequently when our dd was 4, and are back again this year now that ds is 4. We have found that the rides with height requirements are rides that are too much for our kids anyway--they're not really thrill seekers, much to Mommy's chagrin. Rides at MK with 40" height requirements are: Big Thunder Mountain RR and Splash Mountain. Space Mountain has a 44" height req. The Speedway has a 52" height req. to ride alone. Everything else is fair game and tons of fun! At EPCOT, Test Track and Body Wars have 40" reqs., Mission Space is 44". MGM has Star Tours and Tower of Terror with 40", Rock'n' Roller Coaster at 48". The only ride at AK with a height req. is Kali River Rapids, and that's 38".
Have a great time--I think four is the perfect age for Disney!
Barbie