IMO the HPI 3.0 is a junk motor and best served in a landfill, Nitrohawk did point you in the right direction with OS as they make great and easy to tune engines that are very reliable. Anywhere from a .12 to .15 would serve that car fine. Anything bigger like a .18 or more will probably start breaking other parts on the car because of the excess power they produce
Which OS engine do you have? Is you RS4 a belt drive model or shaft drive?
You should be ok with a .12-.15 on stock configuration, the .18 can stress the differentials too much, mostly when used on high grip tracks. In other cases the wheels will just break traction and take some load off the diffs so you shouldn't have any problems.