I don't think the quality will be equal, 28Bsst2 will definitively sound much better.
I haven't tried them but i had 7Bsst2 recently and they were brilliant amps.
Using 4B's in bridged mono is not the best option but it works fine anyway.
I have one bridged for my center channel (that will be replaced soon by a ML 436 mono bloc) and it is beautifull for HT...
If you can't go for a 28, i guess 4B's bridged will do fine... or you might look for 7B's
Personnaly, i would rather go for 7B's.