Took DS3 (nearly 4) last week. He is 41.5". He LOVED, LOVED, LOVED Star Tours! Went on it 3 times (two time in a row, as my friend and I did a baby swap with her little one). 40" is too short for space mountain, but is ok for big thunder mountain and splash mountain. Both of which my son also LOVED!!! We did BTM 2 times in a row (I held on to him for dear life because he insisted on riding both times with his hands in the air laughing the entire way). He got to ride splash mountain 3 times. Twice with me, and once with my friend (baby swap again). We also got to ride Goofy's coaster 6 times in a row...no getting off. There were no lines, and every time we came in to the station, they said we could remain seated if we wanted to go again. He just kept looking at me and saying..."mommy, remain seated..." It was the best. The CMs kept laughing, and even kept count. The only way I got him off was because I could see the rest of the people we were with looking for us and I didn't want them to worry that we had gotten lost! (Well, that and it was almost time for dinner...)
He also loved haunted mansion, pirates, buzz, pooh, dumbo, peter pan, snow white, iasw, the carosel, astro orbitor, the tea cups and "driving" the indy cars (did those twice in a row with no real wait...plus he went again the next day with my friend)! And he got mad at me because I went on two coasters without him (space mountain and rockin' roller coaster). But he survived...and maybe next year he'll be able to ride space with me (I don't think he'll be tall enough for rockin' by then).
Didn't get to Epcot or Animal Kingdom. Also didn't get to do too much at MGM. He threw a HUGE tantrum there. Still not sure what that was about. Tried to get him to do the tower with my friend, but he didn't like the idea if the drop in an elevator. For some reason, regular elevator rides make him feel like he has to pee (no really, it is very odd, and even DH can't figure out why...nor can his doc. Just a strange quirk I guess).
But over all, I was very pleased with the amount of things that he was not only able to ride, but also with how much he loved it all! Have a great trip!!!!!!