Having recently gotten off a 14-day Transatlantic, let me give you the following tips:
1. Be sure that your mail service is cancelled.
2. Be sure someone is there to feed the pets.
3. Be sure that there is no way that work can reach you.
4. Be sure to enjoy.
Seriously, my feeling is that, if you don't mind doing some laundry during the course of the cruise, you can get by on the seven day cruise with the same amount of luggage you took on the four day cruise.
Oh, one more hint. TAKE ME WITH YOU