We're on Millenium in a room that is only equipped for three and we're a total of four including our baby. Celebrity OK's the room for us b/c the baby doesn't need a bed, just a port a crib. Once I called and they said this would work, I checked the various web sites for pricing as if we were 3 and then just doubled the third person rate to get an idea of what our fare would be. When I finally booked the cruise w/ an agent, she told me I couldn't get the room I wanted and I made her call Celebrity directly to explain one member of our group was a baby.
I imagine any quad room will work for your five, just get permission up front from the cruise line and price the way I did.