JHF
Driving Dynamics Pro
I still see the upcoming M335i to be the real competition with S4. 335i is just another 3-series with a different engine. There are much more differences between a S4 and standard A4, to name two things, a proper sport diff and driving dynamics.
Here is one example. Best time both in the tests around the world.
http://fastestlaps.com/comparisons/bmw_335i-vs-audi_s4.html