How can I tell if it's a s4 or s5 just by looking at the motor? No car
Do I need a t2 CAS or can I use a N/A?
Are the coil exchangeable between a t2 and N/A?

 

The AFM and OMPs are the dead giveaways. S4 AFM is large and boxy compared to the tubular S5 style. S4 will have a rod attached to the throttle linkage which goes down to the mechanical OMP on the front passenger side of the engine.

 

CAS and coils are the same between the series and engines.NO difference.

 

This, the S5 afm is a black tube, and the S5 OMP would have an electrical connection.

Another easy check is the Turbo if it hasn't been changed, an S4 has a rather large external twin-scroll actuator attached to the exhaust manifold, an S5 turbo does not have that.

 

fast way to tell the engine series is the thermostat neck on an s4 is 2 bolts(metal).and on an s5 is 3 bolts (usually plastic unless aftermarket).

 

Series 5 has threaded knock sensor holes just above the Trailing plug on the rotor housings, Series 4 does not.

All these methods are dependent to no one changing anything over the past 35 years.