As a first cruise, I think a 4-day is the best of all worlds as it gives a flavour of a port day, Castaway and a sea day. We've done, 3, 4 and 7 day and after our first (a 4 day) we find the more days the better. You are paying for the cruise and ship and all it has to offer and the sea day allows one to experience more of the ship. Just remember a
Disney cruise ship is not a Disney theme park. It will have Disney's attention to detail and their expected service but it is not one ride to the next and non-stop diversion till one drops.
Personally I never thought we'd enjoy cruising and being tied and confined to a ship but 7 cruises later (and an 11 day one this fall) we've changed from over planning our cruises (like a theme park visit) and getting off at all ports to most cruises not doing all we want or have planned, being picky of ports we get off at and enjoying the ship more each time, more relaxed. What you don't experience your first cruise, save for your planned next.