Our first cruise was 3 days. We LOVED it. Enough of a taste to want more. BUT it went incredibly fast. Previous posters have said the same. Since then we've gone on longer and longer cruises.
All that said, 3 days for a 1st cruise will give you either a hunger for more, a love for cruising, or just provide you with a meh feeling.
Our cruise went to the Bahamas as well. We didn't do much there--walked the market by the port and then snorkeled (excursion trip), which included a wild trip to its location. It was the first time also for us to experience being driven on the opposite side of road than USA. Crazy! lol
If you are going on the Dream, you have to know this ship is much larger ship than the Magic or Wonder. LOTS to do with very little time on a 3 day trip. IF you go with the 3 day option, go with relaxing in mind, not trying to cram everything in those few days. It's sorta like going to the parks in that there is a LOT to do and see and little time to get it all in one day-- then at the end of the day...you are dead dog exhausted and flop into bed.
A 4-day would feel like what our 1st cruise did - enough time to explore ship, do some activities on-board and have a day at sea, plus experience the Caribbean/Atlantic waters (so clear and warm), and a private island (Castaway Key is awesome).
Either cruise 3-day or 4-day, go with the idea it will be fast (and fun). And enjoy.
And to mention once again, we loved, loved our first (3-day) cruise. It left us wanting to do it again and again. We were hooked!