Don't believe that "let down" stuff. The first time my kids went, the youngest was 6, and she loved all the other parks after we went to Magic Kingdom first. MK is probably the best place to start for one very good reason. I found that my kids like the shows and characters better than the rides. At MK, even if you get there at noon when it is crowded, you can see the parade, the Cinderella show, the Country Bear Jamboree, Mickey's Philharmagic, and go to a character dinner (book in advance, of course) all even if the park is crowded. Mix in a couple fast pass rides and your 4 year old will have a blast. Second choice would be MGM because there you also have many shows and a parade that can be viewed no matter how crowded it is.