One problem related to power steering fluid has been reported for the 2005 Audi S4. The most recently reported issues are listed below.
I was on my way to substitute teach at an elementary school in santa cruz, CA and so was in the middle of a lot of early traffic at a left turn signal when I heard a very strange rumble. I thought it was another car as my car had just been in for a 95,000 mile service a few weeks earlier. However, part way through the turn I was almost unable to steer the car. I had to fight the car to keep it from hitting a bicyclist and to get it off to the side of the road where it then stopped steering and died. At that point it was blocking two cars in a driveway, but I could not steer it at all. This was on the street directly in front of the del Mar elementary school. I called farmers insurance and monterey peninsula Audi where it was towed. I was told the steering column had cracked and the fluid had leaked out. I was also told there was no way to be able to diagnose this in routine services.