RE: best engine for an RS4?
Given your responses and my knowledge of engines....
It sounds like you want something like a high performance, mid RPM, 3 port engine. The engines that will be the perfect balance of your responses above will likely be somethinf like a Picco or NovaRossi.
Those 2 companies have been in the industry for years, incicating a great deal of experience and ability to R&D on a practical level. They are italian, who by established reputation, have a tendency to construct performance machinery with extensive QA/QC. For this reason, I choose them over some of the newer companies. They are also builders for many other companies (OFNA, Trinity, etc.) meaning that many other brands are de-tuned versions of this brand.
Motors of the .15 size will give you great torque, allowing for gearing mods. (A .12 is more limited to lower gearing due to decreased torque). The .15 will not be drastically overpowered, and give a great balance of practical RPM and torque to get the car moving. Italian .15's usually rex higher than US .15's, giving you more top end speed.
A 3 port will be reasonable to tune. A 5 port may require tuning every time you run, may have 3 needles as opposed to 2, and will undoubtedly increase your frustration factor.
They definitely fullfill the "wow" factor.
A 3 port is not a full blown race motor, so it will have a respectable lifespan.
Recommended:
Picco XP series
Novarossi NS or LS series (LS, if you can find one is INSANE)
Look for something that will spin over 37,000 RPM