Hi, good to see someone else keeping the 4 stud hubs. I have an R32 GTS 2 door here in Oz and have not gone the 5 stud conversion route as there is nothing I am interested pursuing in that direction. And I still have awesome fun with my car as a daily and at track days. Thats all that matters.
Anyway, this site I found very useful, select Brake Upgrades and you get a whole lot of useful info.
https://en.m.wikibooks.org/wiki/Nissan_240SX_Performance_Modification
Funny that this was from the US but helped me do a modest front brake upgrade here in Australia. I fitted s13 turbo calipers and went from the stock 257mm diameter rotors to 280mm. The s13 turbo stuff is a direct bolt on for your fronts. The calipers are still single pot but just a fraction bigger piston so no need to change your brake master cylinder. I currently have DBA (Disk Brakes Australia) T2 slotted rotors (DBA919S) and some QFM pads (QFM DB1232 A1RM) and the result is awsome. I know DBA sell in the US so check that. For pads just look for s13 turbo stuff you can get. Dunno if QFM stuff is available in the US.
You are right about the rears they are not s13. You will find that R32 and R33 4 stud rears are the same but dunno if you can source R33 stuff where you are. Check that link above as I can't remember if it covered rears Unfortunately I have not found rotors from DBA to do my rears Another Australian company, Rotors and Disks Australia (RDA) do rears (part number RDA 7692) so could try ebay??
Anyway good luck with it and enjoy your car.