Normal
It may be because they have already filled their Passenger Capacity quota, even though there are still quad cabins available. Cancellations are inevitable, and since final payment for that cruise isn't due until May, that would be when the actual passenger count may be more accurate. You might try checking back then, or book a quad cabin for two, and then see if you can add the 3rd and 4th later on. There also might be a large group booked on that sailing.
