The title refers to 4 rooms. Alex, Mallory, Jennifer, and Skippy sleep in the living room, because they rented out two bedrooms, but that means there are still two bedrooms available for their own use. No explanation is given as to why they don't use those rooms, although it later becomes moot when a large group of co-eds barge in and stoke Alex's greed into renting out the two remaining rooms and basically taking over the house.