We did Fantasmic Package in January for a 6:30 showing and the latest seatings were at 4:00, regardless of restaurant chosen (that's what we were told, anyway.)
The seating philosophy is based on the idea that all Fantasmic dining package people have to be guaranteed done with their meals and out of the restaurants in time to show up at the theater approximately 60 minutes before show time. So they want to make sure you're all done by 5:15 at the latest to allow walk-time to the theater. So, 4:00 ADR, seated at 4:10, give or take...an hour to order,eat, settle up...you're moseying out of restaurant around 5:15...just in time to walk to the theater for the show.
We didn't particularly want to be eating dinner at 3:50 (the latest available when I called for our package) but Disney Dine person suggested maybe we should do a Fantasmic lunch seating instead (I think we ate at 1:15) which worked out really well. We did that, and then just had a snack around 5:15 before heading into the show. We had a nice relaxing dinner elsewhere at 8:30 that night, after the show.