Any time in March/April tends to be spring break for many western states, so DLR gets very busy during that time. However, as cinna said, the crowd trackers really can't predict with any accuracy, especially since we don't know how the opening of the new Star Wars land will affect crowd levels and movements in the next year. That said, if you're arriving on a Sunday and allowed 4 full days before heading home, that's lots of time to see everything. I might suggest flying in early on a Saturday, spending the first half day at the parks in case the fireworks and shows are only on weekends still (in the past, sometimes it's not a full entertainment schedule until later in the month or spring break season)-- then you can see fireworks, Fantasmic and/or WOC on Saturday or Sunday nights. Then you could tour Sun-Wed and fly home Thursday. If you follow the tips for touring found in the sticky threads above, you should be able to tackle even heavy crowds by getting there early, maximizing FPs, leaving midday for a break when it gets crowded, and then coming back in the evening for shows and rides for which you picked up FPs earlier.
Sundays apparently have been quite reasonable lately, crowd-wise. Mondays are often worse.