My DS turned 4 on our trip in September, and he LOVED Fantasmic! We saw it our last night. He had looked forward to it the whole trip. Sorceror Mickey fighting the dragon was his favorite part.
The only thing he was scared of the whole trip was Dinosaur. We had told him all about it, and showed him pictures, thinking that he would not want to try it, but he was very insistent about riding it, and he hated it.