Ok, I coincidentally went to AMAG today and when I was asking tons of questions in order to prevent to get shafted too heavily (probably futile...) I asked them on the wheels and stuff (I have some S-Line parts on mine myself and was curious on the OEM parts from other models question):
- In Switzerland, the manufacturer of a part has to guarantee that his parts fit a car and are not dangerous...
- In the case of the Audi parts you have on yours, Audi has done this for the models they were from.
- Audi will very likely not give you anything saying that A8 brakes are safe on an A4 as well. We all know they are, but the process to get them homologated is costly and they do not do this for fun (or very rare cases).
- If your rims were from an S4, but the same rims were sold on A4s as an extra, you do not need any paper work at all. I have some that were on the S models, but sold as "S line" options for the common people
- If they were S4 specific, you lost. No chance - except of a "Einzelabnahme" which is so costly that you probably can buy several sets of "S4 style" rims looking identically but made by other manufacturers and homologated for your car...
Sorry, Switzerland is simply not the right country to customize cars...