Are all the rooms the same size roughly?
If pay by room then maybe the biggest room should cost most money, which will presumably mean the couple pay more than 1 person but prob not double.
I think all bills should be split per person though.
Not an easy one to answer as not all situations the same.
Can you come to an amicable agreement amongst yourselves?
I think its fairest to split 4 ways as 4 people are sharing the lounge, kitchen, bathroom, phone, washing machine etc etc Reckon thats more a factor than bedrooms - good luck though.
Agree with buchanan; the old saying about two living as cheaply as one is garbage. Also you need to consider the feelings of the other room occupants; the fourth person will be seen as a freeloader. Not worth the hassle; split everything 4 ways, and keep it amicable.