As someone else stated, book the person now (if there is lifeboat capacity) and worry about where the 4th person sleeps after (s)he is booked. It doesn't matter where that person ends up sleeping and no one will care. You do not need to bring bedding from home, other than maybe an air mattress. Wherever that 4th person ends up sleeping, the room steward will provide whatever bedding you need, whether it's in the 4-person room or the 2-person room.