RC has lots of 4 & 5 nights in the Med, Navigator of the Seas out of Malaga all October 2009 also out of Barcelona all May 2009. Vision of the Seas out of Copenhagen all August 2009, the sun? The same ships also have 4 nights during different months.
There are lots of 4 & 5 night cruises in the Caribbean, Carnival in particular. Go to their website & click 2 - 5 night cruises (over 1200 of them).
If you dont want to travel that far then Costa have loads in the Med.