Some "back to basics" comments:
Secondary ignition faults (plug wires, plugs, coils - everything from the primary connection at the coils thru the plugs) tend to get worse as combustion chamber pressures increase. This often means that the problems are worse at near the max torque RPMs with full throttle, less noticeable at light throttle openings.
Injection problems can vary. If one or more injectors are partially plugged, the misfire will be worse at full throttle. If one or more are fully plugged or non-functional, the misfire will be very noticeable at idle, and less noticeable at high engine RPMs with light throttle openings.
The S4 cars have two almost-independent ignition systems. If one system fails (two cylinders on each side of the engine) the roughness will be very noticeable at lower engine RPMs and heavy throttle opening, almost not noticeable at high RPMs and light throttle openings.
The electronic/electrical side of the injection is often an off-or-on deal. If an injector connection shorts out, it kills the entire system. If an injector connector opens, only that injector is dead, and it is usually dead at all speeds - not throttle or speed sensitive.
Mixture problems can be caused by vacuum leaks ("false air" - that is, air that is not measured by the MAF); or by sensor problems (Temp II being most common).
If the Temp II sensors (there are two in the sensor - one for injection, one for ignition) fail, the result depends upon which side fails and how it fails. If the LH side fails open, the fuel/air ratio will be very rich, with poor or no running when the engine gets warm, and very, very hard starting when warm. If the LH side fails by shorting, the engine will run very poorly when cold, and will be hard to start cold. If the ignition side fails, there will be a slight loss in power, but much less effect than if the LH side fails.
Oxygen sensor failure can seriously affect running, but if you just unplug the sensor (the connector is the black wire that runs across the floor at the bottom of the Central Electric Panel), the car should go to "open loop", and should run pretty well.
There are, of course, mechanical problems that can affect running. You might need to pull the belt covers and check the camshaft timing - that is, make sure that the crank and both cams are on the timing marks. A compression test will usually let you know if there is a problem with cam timing.
