Ok so been looking around and cant seem to find anyone that has done a full motor build or and company that offers a stroker kit. I know u can get 60-70whp out af fully built heads, but is it possible to make 400whp or close to it from a full engine build.
You may want to get in contact with APR about this or even audi. The 4.2 is good for a lot more then 4whp fully built but it could be very hard to find anything for it at they are still using that motor in GT2-3 class cars. But fully built i think even if you keep it at 4.2 and dont up the displacement you could see more then 400. The R8 4.2 makes 430 from everything i can find on it. So that should be your answer right there.
The R8/Rs4 engines are over 420-430 crank. You are talking different cams/intake/throttle body, not sure on the bottom end internals, fueling is different, etc. Increasing displacement is going to be harder due to the fact that is an alloy block and if you have to clearance it too much you will weaken it. I don't know how cost effective buying parts from audi will be. I'd consider the JHM supercharger, probably much cheaper way to over 400 crank hp.
Just thought I would bump this and reply since I've talked to JHM about this.
I plan to get thier supercharger kit when it comes out and maybe have them build a motor. They think the motor is good for lots more power as well. I have a sneaking suspesion we will see a-lot more 500awhp S4's soon.