Like Sonny pointed out, it may not have anything to do with "fitting" or not, the cabin you are in may not be designed to sleep 4. The 4 people cabins have pull down bearths. Check with your travel agent and find out before you say yes or no. If your cabin does not sleep 4, then the 4th may have to pay the "extra" to get you into a 4 person cabin if you want to go that route. If the ship does not have a 4 cabin availble,the only other option is moving your 3rd into a Cabin with the 4th but then the 3rd is going to end up paying full price.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013