@Bazza, this past summer I had your same scenario. The only problem is whether there is still lifeboat availability for a 4th person. Even if there are still 4-person cabins available, the lifeboat availability may already be maxed out. We booked this past summer's cruise several months before the nonrefundable deposit thing came into play, so our TA advised to book the 4th person and cancel later (which we ended up doing).
Again, that's not related to the deposit, but it's another factor.
kellie in texas got a reaction from Bazza in specific nonrefundable deposit question