Hi Bobby, there was a C4/rpm thread a while ago, but it seemed not many people have ridden both.
www.seabreeze.com.au/forums/Kitesurfing/Review/RPM-vs-C4-any-suggestions/?SearchTerms=c4,rpmI have C4s but only logged a couple of sessions before taking a break for surgery. The C4s are smooth, responsive, stable and clearly the construction is industry-leading.
I've not ridden the rpms so can't comment on performance details. Seems there were some durability/functionality issues with the bar/trim-line/cleat.
Clearly they're both performance kites. You'd have to demo yourself as to what suits your style.
Be aware that if you're coming of bows then they may take some getting used to before you appreciate their strengths.