I've got a s5.
I've got a s4.
The s5 is my baby and the s4's engine was block welded and blew after two days of me owning it so I'm parting it out/ using a lot of the parts.
My s5's fuel pump relay (one under the steering wheel, right?) was fried and I was going to use the one from the s4, but was wondering if they are interchangeable.
I'm also going to use the plug on the top of the fuel pump so I can get rid of my ghetto rigged fuel pump switch and so I can actually fill my tank all the way up without it leaking out the top.
Is that plug the same on the s4 and the s5?
Also is the reverse light plug on the transmissions also interchangeable.
My s5's tranny got stuck in reverse and while fixing it I broke my reverse light plug...
More questions to come, trying to restore my car.

 

I don't think so. The s5 directly controls the fuel pump from the ECU, the s4 does not.

 

From looking at the wiring diagrams for both the S4 and S5 it appears it would work. If the top left wire has voltage w/key to start and it causes the Blue wire (top right) to have voltage that is good. If the top center wire has voltage w/key to on that is good and if the top right wire, Blue, has voltage with the engine running then that is also good. If it does these three things then it should work.

 

If you mean the 5th gear/reverse switch, then yes, you can interchange them. If you mean the plug itself, I would say so as well. If not, you can just crimp/solder on a couple new connectors.