I haven't any experience with the A4's but am about to buy my third Audi (a used S4 Avant) and can comment on Audi reliability in general.
I bought my current ride - a '91 90 Quattro 20-valve - six years ago with 95,000 mi on it. I think I was the second owner, and it had been pretty well cared for. It now has 270,000 miles on it and except for a failing transmission, it is still in excellent shape; in fact, it still feels more solid than our newer Honda, Toyota or Ford.
My first Audi was a '90 100 Quattro, which I bought with 28,000 miles and kept until it was totalled by an oncoming pickup at 230,000 miles.
I kept pretty close to the recommended service intervals, and do most work myself. Parts costs are pretty comparable to Honda's and from what I hear, dealer service is just as outrageous as for the Japanese makes. However, the quality of the OEM Audi parts is higher than most, and, for me at least, last a very long time. In addition to the original major mechanicals, my 90 still has its original a/c system (converted to R134a), power steering, master cylinder and front calipers, alternator (I replaced the brushes at 200K) - even the radiator hoses are original and in good shape!
There are some good Audi enthusiasts on
the AudiWorld forums if you want to ask questions about the A4.
Happy hunting.