1997 Audi A4 no power

Your car is in "limp" mode as it has had multiple misfires in multiple cylinders. It sounds more like a fuel air or tune issue, which could result in a bad catalytic converter though you won't know until the rest of the problem is fixed.

Whats going on in a nut shell is your car had something go wrong that caused a rich mixture of fuel to air, this caused detonation and misfires and caused the engine to retard timing, it also sounds like this went on for a while and the timing is now so far retarded that the system needs to be reset. First thing to find out is if someone turned up your boost beyond factory spec, this can easily cause this problem, next thing to do is repair the ignition system with new plugs as they are most likely a lost cause at this point. At the same time, check your plug wires and boots. Before all of this I would have the codes read again by another party who is less interested in doing work for you, like an auto parts store, so that you can get a less biased opinion.

After all of that you may end up replacing the cat as it is often fried due to rich mixtures.