I have been hearing for a long time that there will be no RS4 for the B8.

All their attention is going to the RS5.

In regards to the 4.2 ... it really isn't that heavy - all aluminum so no need to complain. If it was too heavy Audi wouldn't use it in their flagships A8 and R8 now would they? No. Weight can be a little bit of a factor but not that big of a deal. On a similar note, I believe the main reason for the switch to the supercharged 3.0T is fuel mileage... and they didn't go back to the turbos because of poor reliability. You can tune suspension to accommodate for a hair more V8 weight.

THERE IS NO REPLACEMENT FOR DISPLACEMENT.