Simple answer is DCL won't book you in a room with more people than it can sleep. If you want to know for sure call DCL or go on the web site and try to pick a date far enough out that you can choose that room.
We were just wondering about adding another person to our cabin, whether we would need a different cabin or if the one we are already assigned would sleep four.
Depending on how full the ship is and how soon you cruise you may or may not be able to add a person to the room even if it has accomodations for 4. I would check with DCL reservations. If your cruise is a long way off, I'd get the 4th person added ASAP to get the best rate. and then cancel later if necessary.
Also, if the 4th person is a child or teen they will only allow so many of each age group on board. There could technically be room on board, but not in a particular age group.
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