If you purchase a used G4 board, you either get 1) a 17.5psi limit 18ohm board, or 2) a 19.5psi limit 15ohm board. It depends on production dates, or if the original owner did a 18ohm to 15ohm modification.
5soko gave the best reasons for getting a G5 board. Add to that, the G5 also had built-in CPS module.
'15 IS350 F-sport Ultra White/Rioja Red
'09 E90 M3 7-DCT Interlagos Blue/Novillo Silver
[MS Pullies|K&N drop-in|Test Pipes |Evolve Stage II tune|Evolve Servotronic Map|Evolve M3 GTS-DCT file]