Four-Stroke engines are nothing close when it comes to high R.P.M of Two-Stroke engine. Four-Strokes engines may offer huge amounts of torque, better fuel economy, and cleaner running ,but will never reach the required R.P.M needed to make the car as fast as a Two-Stroke. Most Four-Stroke engines rev between 18-20K while Two-Strokes can rev upwards of 30K R.P.M.
Theirs simply no market for the four-stroke engines now thus meaning no improvements ,but when the day comes where four-strokes become popular again look out.
As far as trying to custom lengthen a rs4 chassis it's already been done, I would just purchase a HPI Super Nitro RS4 kit and work up from there with the help of "Wolfpack Radicals".