I hope it won't be too crowded with only two sittings for dinner.
Don't be confused by the four seating thing - it was really still two seatings but they were staggered by a half-hour for ships with two main dining rooms. One dining room did 5:45 and 8, and the other did 6:15 and 8:30. Some ships never had four seatings if I remember correctly. Carnival hasn't always done the four seating arrangement. It was started a few years ago but I guess proved to be more of a hassle than it was worth, and now they are returning to the "usual" way.
Actually Carnival is also testing anytime dining on some ships too.
As for your preference, the only thing you can do is go to the dining room manager on embarkation day and see if he can accommodate you. They will be available at the dining room (often the entrance to it) or at some other place which is listed in your Carnival Capers which will be in your cabin.
Being seated with others is usually a high point for many of us cruisers. It is nice to meet others and share the cruise experience with them. We had some wonderful tablemates on our cruise to Alaska last month. We ended up hugging goodbye on the last night.