Do S4 rear axles fit into the A4 hubs? Or do I need to get S4 wheel bearings and flanges as well? I'm fixing to find out the hard way soon... ;-)

I currently have the flanges in the rear dif swapped to A4 flanges, and have been running A4 axles for 3-04 years without any problems. I just want to upgrade to the stronger axles, as I'll be running more HP/TQ. That, and I think the A4 flanges in the S4 dif are cause for a certain noise I have as well.

according to my copy of ETKA, there is only one wheel bearing housing for the A4 and S4. Only one hub part number and one bearing is shown as well. The axles are, of course different but the outer CV joint, hub assembly, wheel bearing housing and bearing all appear common. The upper control arm shows a different part for the A4 and S4 and a part change based on VIN for the S4 part. I wouldn't think you'd need bother with the upper control arm.
All that said, if I was looking to beef up the rear axles, I think I'd look at Raxels before using S4 parts.<ul><li><a href="http://www.raxles.com/default.aspx">http://www.raxles.com/default.aspx</a</li></ul>

I won't be drag launching my car, so I don't think Raxles will be for me, but thanks!

The a4 rear axels should be strong enough!

I am running a rs2 box and rs2 rear diff now,
and I am running the rear diff locked on dry road with slick tires!! Thats about the worst thing you can do for the rear axels, but they are holding!

And for for axels, and whats breaking, the axel itself never is the problem, the CV joint usually is, it'll brake in one end.

And yes he is running a 6sp box and matching diff...

well, I guess I can't say that really. My rear dif has developed a noise, and when I swap it, I want the correct flanges in it. I bought the S4 axles, and was just curious as to how simple or complex it'd be to install them. but the axles never gave me any worry.