I've had my 04 S4 Avant for almost 2 years now.
The biggest issue that scares people away from early 04 S4s is the timing chain or timing chain tensioner broke. Later versions were stronger, but on early cars this was a weak point. I have an Oct 03 car, and the chain or tensioner on my car was never replaced. Since it is on the back of the engine, you have to pull it out to replace. $$$$
Other than that, I haven't heard of anything being less reliable than other Audis. MAF, electronic gremlins, interior switches, power windows, etc. But driveline is pretty solid.
I've had two problems: window switch broke, and the secondary air pump broke. No other problems for 20 months or so and 15k miles. (we don't drive much, I don't commute with it)
Terrible gas mileage. Like an SUV. But, I'd rather spend the gas in a nice sporty car than a big truck 🙂
I love the engine sound, like the looks, and it drives very nicely. IMO, I've decided that I don't like having over-abundant power. I'd rather drive 170hp like a race car than drive 340hp with a light foot. But the highway entrance ramps and occasional other spots are fun 🙂
I'm thinking of selling mine. Mainly want something new. But I can't think of anything else I'd prefer, so I'll take my time.
Favorite moment:
One time when getting on the highway, my 6 year old daughter said from the back seat: "Here comes the fun part."
Ha!