Hello, Well, you are not in Key West very long. We left around 2:00pm after an arrival of 7:00am. But I think you are there long enough with plenty to do in the time there. Key West in a small concentrated area, so you shop, check things out, and get back on the ship.
Honestly, we didn't do much in Calica, but I did hear raves about the cave snorkeling. I would book it based on what people were saying. Since I have tubed in caves in Belize, sounds awesome. Since I waited until the last minute, everything was sold out. I mean everything, so book early and only with CCL. My opinion. The Caves, all snorkeling, and dolphins encounter go quick. My son is a little fussy about adventure, so I had to wait as see what was available. That worked out well for me...LOL!!
4-days is a little short, but I think perfect for a quick get-away. There is a formal night and a midnight buffet all on the second night. Quite a spread too. Since these ships are older, I found myself missing the Supper Club and a balcony, but made do with oceanview.
I hope I was able to help and answer any questions you may have had. Food for thought, make sure to bring those rubber-soled water shoes if you decide to cave-snorkel. Take care!!