If you consider that your cruise starts at 5pm on embarkation day and ends at 8am on the day you leave, I made the following calculations:
3 day cruise - 63 hours
4 day cruise - 87 hours
7 day cruise - 159 hours
So that means you achieve 38% more cruise time with the 4 day over the 3 day.
The percentages for the 7 day cruise are:
7 day is 152% more than the 3 day
7 day is 83% more than the 4 day
Based on that criteria, the 4 day is a better experience for a little more money.
Based on personal experience, we enjoyed the 4 day cruise with a day at sea much better than the 3 day. It's a different experience to be cruising during the day rather than being docked at port.
hope that helps