i just heard from the folks at porterfield. they will do custom r4s pads for the f90, same price as the ACG pads. they also said they plan to offer pads non-custom which will be a little cheaper but that they wont go into production until june or july. i had the r4s on my f82 and they were great.
https://www.porterfield-brakes.com/c...reet/R4-S.html
https://www.porterfield-brakes.com/i...6_02_45_30.pdf