Steven,
Veltyen has this one dead on. Take stateroom cost and space from whichever rules set you're using, figure that is the cost/space for long term habitation for two sophonts, and do the math; i.e. multiply by four to get your 8-man bunkroom.
Don't forget, stateroom sizes are meant to include space for passageways, lounges, and the rest also. It just isn't 4dTons alone.
Have fun,
Bill