What the of center differential is used on teh A4 2.0, A4 3.2, S4, and RS4.
I know that the torsen diff would mean its permanent AWD while the Haldex diff would mean its only part time with torque transfer to the rear only when slip is detected in the front.
Audi's website is useless and the only other way I can think of to find out is go under the car and check it out myself which would be really really uncomfortable as its winter right now.
The S4 and RS4 is definetly permanent 4wd so it should be safe to assume they use torsen diffs. I'm unsure for the regular A4 2.0 and 3.2 quattro tho.
P.S. it's good to be back