I would do the 3-night first, get most of our must-dos out of the way, and then kick back and relax on the 4-night. I would much prefer a 3/4 night B2B that stops twice at
Castaway Cay over a 7-night cruise that only stops there once. We've sailed a 7-night and weren't impressed with what the Caribbean islands had to offer. But Castaway Cay is a gem. We also have sailed a 3-night and didn't find it to be any more "rushed" feeling that the 7-night. It was just shorter than the 7-night, that's all.