Carnival has a variety of 3 and 4 day cruises. It is according which port you want to go out of: the "Fantasy" goes out of Pt. Canaveral for 3 and 4 day cruises to the Bahamas, The "Sensation" goes out of Tampa on 4 and 5 day sailings to the Western Carribean (Cozumel and Grand Cayman), the "Fascination" goes out of Miami for 3 days to the Bahamas and 4 days on the Western Carribean, and the "Imagination" goes out of Miami on 4 and 5 days to Western Carribean. The "Celebration" goes out of Galveston, TX on 4 & 5 day cruises to Western Carribean and so does the "Jubilee". Out of New Orleans the "Holiday" does 4 and 5 day Western Carribean and out or Los Angeles the Ecstasy does 3 & 4 day cruises to Baja Mexico. So there are a lot of choices out there, some of these ships are repositioned throughout the year, but its according to when you want to go and what port you want to leave from as to which ship you will be on.
Hope this helps. All the Carnival ships are great, I have personally been on Jubilee and Carnival Pride they were as different as night and day. Its just what ever you are looking for.