The buffet option is only offered on the 7 day cruise.
Your alternatives to main dining on the 3 or 4 day is limited to the counter service food on deck 9 (pizza/hamburgers/chicken fingers) or room service.
Keep in mind though that the dinners on the 3 and 4 day are MUCH less formal then a 7 day cruise. Triton's would be your most formal, and it's suggested to wear a jacket.. but you could show up in a polo and khaki's and be totally fine.
I wouldn't miss the main dining.. it is wonderful. Bottom line, wear what you will feel comfortable in, you really don't have to worry about getting very dressed up. Khakis and polos will be fine.