We were there April 20-25 this year. It was pretty crowded on the weekend, especially Sunday at MK. MK was having their high school party nights that weekend, and the park closed early on Friday and Saturday. Sunday was the only weekend night they had Spectromagic/Wishes. The place was PACKED.
If you do go in April, I would recommend getting park hopper passes, that way if one park gets crowded, you can always go to another. This works especially well for MK and AK, which tend to be less crowded in the morning, but fill up in the afternoon.
The nice thing about April is the weather. It is warm enough to go swimming and wear shorts, but it isn't too hot and humid yet. In the two times I have been in April, we only had rain one evening.