You don't mention what type of problems you are having. You can't find engine mounts that work? Not lining up right? Not staying mounted properly?
I'm not sure, but i believe the rs4 3 was a rtr. In which case you didn't get any engine mounts with it and probably came with their nitrostar .15. Yes?
I am not well versed in this model exactly. Several buddies have rs4 versions and all have put other manufacturers engines in them. My son has the MT2 and I am in the process of building an MT Racer. After looking at HPI's website, all appear to have about the same platform on which they were made. Someone I'm sure will correct me if I am wrong.
I am putting an O.S. .18 cv-r in mine. It screwed right in with the mounts they provided. Here is the line just as it reads in the manual:
A863 2 Engine Mount (HPI, OS) 4.00
The price might not be the same. It's from 2002, but I hope this works for you. I haven't ran mine yet, (still waiting on some parts) but it looks to be lined up good. Good luck.
You're not having fun until you break it.