Hi SoundGame,
I'm interested in the answer to this question as well. Since we haven't heard from James I'm betting his answer is going to be if your room size and speaker load are proportionately lighter/smaller and in the 4B3's wheelhouse the answer will be yes. He has always said the amp hierarchy at Bryston is not about good/better/best but about application i.e. what size room do you have to fill and how tough are your speakers.
I just replaced my 25 year old Infinity 9 Kappas with a new pair of Focal Sopra 2's. The Infinitys dipped to .8 ohms at 30 and 340 hz with nasty phase angles. the Focals are an easy 8 ohm nominal, 3.1 ohm min and 91 db sensitivity. This is why I'm having the same thought. My 7B SST2's think they have died and gone to heaven 
Yes, my guess is that the new 4B3 will outperform the previous 7B-SST2 when operating within its optimal power range ie. dynamic demands not exceeding 300 to 350 and speaker load being above 4 ohms.
My room is 13' x 18.5" x 7.5" and acoustically treated with bass traps, diffraction, absorption, including shag carpeting. My speakers are not a well known brand in North America but come from a German manufacturer: Aurum Montan VIII by quadral and are full-range, extremely transparent, utilizing a magnetostat tweeter and exotic driver materials (ALTIMA - aluminum, titanium and magnesium); I also have a pair of KEF LS50. Both of these speakers don't present demand too much demand on my 4B-SST2, yet still I was considering the 7B-SST2, since it does have the BIT transformer configuration and more dynamic capability (my speaker can handle 500 watt peaks). My expectation is that the 7B is more impervious to less than optimal power at source - providing more consistently better sound in the sweet spot. I expect that the 7B-SST2 would outperform my 4B-SST2 even on my nominal load when listened to at the sweet spot. I expect blacker backgrounds and therefore more fine details as well as even greater ease - though all these would likely be small improvements and perhaps only noticeable on A/B/A comparison.
With the new Cube series, from what I'm hearing, I expect the 4B3 will outperform my 4B-SST2 by a greater degree than a pair of 7B-SST2 - looking to James to share his thoughts. I'm also guessing that the gap between the 4B3 and 7B3 remains as it was with the previous generation, so in a couple of years, even if I go to a 4B3, I may likely be back to questioning the merits of a 7B3 in my system.
I also guess that there is a greater difference between the 4B and 7B then between the 7B and 28B where power demands are adequately handled by a 4B.
