Hi 914nut. I've been going back and forth between these two amps myself, albeit for a subwoofer application.
Jameswei is correct, there's a switch on the back of the 7B that allows you to use the amp in parallel or series mode with just one speaker being driven. Some 7Bs have two speaker terminals and some have four, but from what I've seen they all have the parallel/series toggle switch.
My quandary was whether to get the 4B for it's ability to power one or two mono or stereo subs, and also have the ability to be bridged. It'd be nice to have a stereo Bryston around for fun as well. But, my main reason for looking at these two amps was for subwoofer use, and after corresponding with Bryston I've decided to get the 7B.
Basically, the reasoning is that while the 4B is rated at 800 watts into 8 ohms bridged, it may not be meant to run lower impedances in that mode. Where as the 7B can handle just about any load down to 1 ohm, and can be tailored by the switch on the back for that. Since you're running speakers that are problematic impedance-wise, that would be something to consider.
The person/dealer I bought the 7B from notified me that it had started smoking when he checked it. He sent it in to Bryston for repair and a full check up, and lent me a 4B during the interim. To reduce any risk since this is a loaner amp I'm not running it bridged, but let me tell you it's a potent amp even in stereo.
I hope this helps. Good luck!