About the cruise...
We have the 4-day planned... I too wanted to test-the-waters so to speak. A full seven days did sound a bit long. But now, having researched all of this, and reading the Navigators on what all is available on the cruise - I don't know how we will be able to do everything we want in 4 days!!! I remember a recent thread on this subject, where nearly everyone said, "Three days is NOT enough" - "Four days are better" - and "We turned around and booked a seven day!". I decided that I would have to have to book at least a 4 day. On our sailing date, it was only a tiny bit more cost than the three day, too. How could I not!
Haven't been-there-done-that, but I would personally go to the Parks first. Saving that excitement of your first cruise for last. Also, after enjoying the cruise - especially with the last two days being pampered at Castaway Cay and At-sea - Then we will be able to go home without being frazzled from the Parks... Feeling like we have had a real break, a wonderful vacation, and maybe not needing a whole week to recuperate!
PS: Did you have a good TA for your previous Disney trip(s)? A good TA can make sure all of your arrangements work smoothly with the Parks and the Cruise. And may be able to save you a little by making the arrangements separately, using specials, etc.. instead of simply booking the package.