I'm confused... You mean a 911 Carrera 4, a 911 Turbo, or a Carrera 4 with a turbo kit?
The 911 Turbo was never given the "Carrera" denomination... To my knowledge at least.
Anyways, what year? 911's vary drastically in performance depending on the generation (993, 996, 930, etc)...
Give more details on the specific cars involved to help people determine what the S4 will need to keep up/outperform a 911.
Once again someone tries to compare apples and oranges.

There was that other thread regarding the S4 vs Z06... When is someone going to start a thread about the S4 vs the STi?