The 4/night cruises do not have a "Formal Dress" or "Semi-Formal" dress up night. Only the 7/day and longer cruises have those. You will have one "Dress-up" night on the 3 and 4 day cruises. I've been on many Disney cruises and I'm sure that khakis and a polo shirt will be fine if your husband insist on it. Other people will wear the same attire and as someone pointed out, Disney will not prevent you from eating in the dinning room if you don't feel like dressing up. However, when I'm on a
Disney Cruise my husband and I like to get a little dressier for the "Dress-up" nights. He wears a suit and tie, and I like to wear a nice dress or dressy pants and top. We like to take advantage of dressing up nicely on a beautiful ship and having a lovely dinner together. We feel that we can wear our everyday clothes at anytime, and we do, all the time at home. So we like to make our cruise "special" by being dressed on the "Dress-up" nights and more formal nights too. However, it's your choice. You will see many people dressed up in their best on "Dress-up night, and other's not as much. The most important thing is that you relax and enjoy your cruise!