When my DS was that age (he was 3.5, but under 40 inches, alas!) he LOVED the kids play area at Mission Space. Looking at all the cars on display at test track was a big hit, but that could have something to do with the fact that his Grandpa is a chevrolet dealer, LOL. He was uninterested but tolerant at the Land (we rode the boat ride), and wasn't as interested in the Imagination pavilion as I thought he would be--he *really* didn't like HISTA.
In the World Showcase, it was hit or miss. He liked the Mexico boat ride (and this was before it was redone--I think it's probably better now). The China and Canada movies, not so much. He really liked Norway, because he got to eat lefse (we're Norwegian by descent and that is a christmas time treat for us) and enjoyed the boat ride. He even liked the movie at the end! Also, and this was a big surprise--he loved Illuminations (I thought the fireworks would freak him out) and we went back a couple of nights later to see it again.
We didn't do everything that trip, but it was an awful lot of fun. I hope you have an equally good time.