We were there for April vacation in 2004 and 2006. 2004 did not coincide with Easter and 2006 did. I can't really say that there was a great deal of difference. How much more crowded than 'really crowded' can it be. I did seem to notice more New Englanders in 2004 than 2006, probably because there were people from a greater range of areas. One nice thing about the past trip was that the parks stay open late and open early. Because you will be there after the main holiday peak period, the parks may start to cut back on the hours already. In 2004 that was a problem, especially at the end of the week. The MK had grad nights at the end of the week we were there, so it closed early and AK also closed early. The worst night we had was when we went to DTD on one of the grad nights. Since the MK was closed, AK was closed it seemed that everyone was in DTD. Couldn't get into stores, or restaurants.
But in the parks, have a plan, make use of the EMH early hours when you can and enjoy the warmth, it will hardly be warm here at that point in April.