Originally Posted by danger

If you book the four nights separately, it will be treated as a separate reservation, up until you check-in when, one way or another they'll be merged (either the hotel will link them or you'll have to check-out of your room after the third night and check back in).

If one couple backs out of the four nights then, yes, you'll have a problem because you'll have to cancel one room. If one couple backs out of the (first) three nights, the BRG goes out the window as you'll have to change the reservation to whatever the current rate is.