are the parts between the reguler rs4 and rs4 pro compatible?

It depends on which parts, if i remember correctly, one has a rear belt drive, and one has a gearbox. I think the suspension is pretty much the same though

The front suspension is a little different, the RS4 had no C-hub carriers. The upper links had a tendency to break, so they switched to a different system that uses carriers, a different arm and a different upper link.