I'm not sure about the details of your particular situation, but if you had any other balance on the card when you make payments it goes to pay off the 0% interest first, then it goes to the higher interest rate balance.
For example, say you had $1000 balance on your card. Then you put the $4000 cruise on it. When you make a payment, say $1500 it would pay off some of the cruise balance and you would still have the $1000 original balance plus $2500 cruise balance. Only after you pay the $4000 cruise off would you begin to pay your original $1000 balance off. Therefore, you were paying interest on that original balance all along.
I hope that makes sense. I don't know how to explain it any better. So basically, if you have any sort of balance on the card, or may any other charges, the 0% interest doesn't mean anything unless you can pay both your new charges AND the cost of the cruise off in the first month.
I am not sure if any of this applies to your situation, but thought the information might be helpful to you or others.
Maggie