Hi! It's true that both rotors are 256 mm in diameter, but the 4kq ones are 20 mm thick, while the 90q are 22mm thick. So you couldn't use the 4kq rotors on a 90q and vice-versa. I hope this helps, Mihnea _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp