You can't go wrong in Magic Kingdom at that age. At age 4, our son was above 40 inches, so it really opened up a lot. He is a dare-devil and a "boy's boy." His favorite rides were Big Thunder Mountain and Mine Train. He liked the Dumbo play area much more than the Dumbo ride itself.
His 2-year-old brother loved Small World, the Carousel, Dumbo, Tomorrowland Speedway, Buzz Lightyear, and Jungle Cruise. Our 4-year-old asked, after riding TL Speedway, "Why do they call it Speedway, it's not very speedy?" They both loved Winnie the Pooh, Peter Pan, Under the Sea, and Tea Cups. They both also really enjoyed the raft to and from Tom Sawyer Island, and running around over there. With the boys running around, my wife and I found the island tiring, and not the break we expected! Mickey's Philharmagic was a nice break for everyone. Neither of the boys liked Pirates of the Caribbean very much - our boat got stuck for awhile, which didn't help. They loved sitting in Gaston's Tavern. They did not like any meet and greets - we did two and then cut those out of our plans. On our next trip, we won't do any meet and greets; we can get plenty of good pictures without meeting characters.