s4 and s5 fuel intector
 

i was wondering if you can use s5 fuel injector on a s4 motor...???

Nope, the connectors are different and I believe one is high impedance and one is low impedance.

S4 injectors are low impedance, so there is a resistor pack in the wiring harness. It's possible if you use the S5 harness for the injectors and then use the high impedance s5 injectors - the ECU will drive them just the same, and they are the same physical size.

Its a lot of work...

Actually, only early s4's are low imp with the resistance pack. From 87.5 on, they were all high imp. They still have the different connector, though.