My R4S pads wore a deep groove into both rotors along the outer edge of the inside braking surface. They were brand new Brembo rotors, & this occurred aftr ~3mnths of use. The damage was in the exact same spot on both sides of the car.
I took the pads back to Porterfield [I'm local], & showed them the parts & they could not explain the issue. So, they ground the pads flat, & I tried them again on a 2nd set of new Brembo rotors...same problem.
Aftr that, I turned the 2nd set of rotors, & used stock pads. The rear R4Ss have worked fine, & they're still there. I can't believe that I'm the only one who's seen this occur.