RE: best engine for an RS4?
This depends entirely on WHAT YOU WANT TO DO WITH THE CAR.
Do you want top speed?
Do you want ease of tuning?
Do you want your buddies to say "ooooooh"?
Do you want big smokey burnouts?
Do you want ridiculous power?
Do you want to get an economical engine?
Do you want an engine that will last a long time?
Answer all of these questions, understanding that most do not go hand-in-hand (EX: ease of tuning and high RPM are not coming from the same engine. Nor does a cheap, powerful engine exist....)
Once you indentify your goal, suggestions can be made.