s4 (series 4) rx7s were made from 86 to 88, s5 (series 5) rx7s were made 89 to 91. there are subtle differences like interior, trim, bumpers, tails, and other shit, but the main thing is mechanical. the engine mounts were placed differently, harnesses are different, ecus, internals on the engines, blah blah blah.

to just keep it simple, stick with the same s4/s5. trust me, youll save a ton of headaches when it comes to wiring and everything.