1. Common problem with Audi concert stereo with tape deck, the volume goes crazy when you turn the dial. It's a problem with the RAM. Need to replace the stereo this will happen to all units eventually, there is no fix from Audi.
2.Central LCD shows lines and is unreadable. Need to be replaced. Due to issue with the connections inside the LCD
3. On early S4 models the turdo's go at around 70,000miles. Expensive as to replace the bottom turbo you have to take the engine out. This is a common problem with the early models. (Audi have now dropped the twin turbo from the S4).
4. The exhaust manifold is twin skinned this brakes and caused a rattling noise at low rev's. Expensive to fix as it requires the engine out can be 40hrs labour. If your lucky the rattling noise may be from a lose heat shield that can be reached with a long cresent.
5. The ABS light can keep comming on due to a faulty sensor.