As others have said, there is no Formal NIght on a 4 night cruise, there is a "dress up" night. It seems that people (well most people) understand formal night, but dress up confuses them and they think if they wear a T-shirt with a pocket they have acheived the goal of dress up night.
My point is, you really don't have to worry about dress up night. I started with slacks, a dress shirt, tie, wing tips, and sports coat for dress up night. Last cruise it was boat shoes, kahkis, a dress shirt, and a sport coat. I think I was still more "dresssy" than about half the people there. On a 4 night cruise you really dont have to go crazy with packing.
If you choose to go with a sport coat, here is my tip of the day. They will clean and press your sport coat on board for something like $10 or so. I would pack it in my regular suitcase (no need for a garment bag) and as soon as we unpacked I would sent it to the cleaners. The next afternoon it was hanging wrinkle free in my stateroom.
-dave