Actually, he never asked about that. Regardless, it wouldn't make sense for Bryston to offer the bridge option if the amp couldn't at least handle a 4 ohm load. Most speakers drop down into 4 ohm territory. The company prides themselves on the fact that their amps can power just about any speaker.
And I agree with ZD that the preponderance of credible anecdotal reports is that the majority of amplifiers will not sound as good in bridged mode as in stereo mode. Probably even when driving 8 ohm speakers, which would be seen as 4 ohm loads in bridged mode.
ZD, I understand he didn't ask about that. I just wanted to let him know that he might want to look into that aspect of bridged amps.
Almarg, there's exceptions to the rule. Accuphase amps come to mind. Thanks for answering my question on which Ohms a bridged amp sees.
As far as power is concerned, I believe he's looking for sound equivalent to the 28BSST2, and hoped the 4BSST2 bridged will provide similar performance. As far as too much power is concerned, imo it's better to have it rather than need it and not have it.