If you had an S4 And an S5 Motor sitting side by side, how would you tell them apart. And lets say the motor I just rebuilt was an S5 and the one in the car is an S4, How much trouble am I in.....

 

Well if the intakes are off of them and your looking at just a block the s5 intake ports are a slight bit larger I think.
I also thought the s5 motor used a electronic oil metering pump over the s4 mechanical.

As for hooking them up, you need to make sure the counter weight you use matches the rotors. Other than that, I see no major problems but I am no expert.

 

weight them, s5 will be lighter.

 

the s5 intake manifold is different