Hey, I searched the forums to no avail. I'm looking at Nex SS coilovers, which they make for the a4, but no listing of s4 (http://www.prostreetonline.com/buy/nex_ss_coilovers/). I emailed pro street online asking if the spring rates would support the weight of an s4, but they just sent a general email and didn't answer my question at all. Now I come to you, will a4 coilovers work on a 2000 s4? hmm

 

The spring weight on the front of the car is different between an A4 and S4, it would fit up but you wouldn't be getting the best handling out of the car.

 

It's unlikely that'll it'll work WELL.
With the S4 being heavier, you'll have a much greater limitation to the height adjustment capabilities.
The rates won't be quite right so there's a good chance the car won't handle very well.
And you should also even double check that they have a kit for the Quattro. I've found that most of those cheap no-name kits only are available for FWD's.

My recommendation would be to stick with some well known brands, H&R, KONI, Vogtland, Eibach, Stasis. You'll do yourself a favor with a kit that's designed for the car.