Hope this helps a few people, i have always wanted a quad exit exhaust on my audi but wanted to keep to genuine audi parts.
Heres a few steps on how i have gone about it, hope it helps any one else wanted to do the same....
Remove your existing exhaust, 2 x rubber mounts on the back boxes and 2 x in the middle under the prop shaft ( if your car is quattro like mine ). Also undo the clamp connecting middle section to front pipe. ( 13mm socket size ).
Sit the s4 exhaust directly on top of the original diesel exhaust, then where the two pipe meet, mark on both exhausts. This is quite important as if you get this wrong it wont line up when refitting the exhaust. I made sure that the rear back box mountings lined up directly over each other there for the exhaust must be the right length when cut.
Once you have cut the s4 exhaust, fit to the car and just make sure it does sit correctly under the rear drive shafts. The s4 rear boxes sit perfectly into the area the original exhaust came out of so no need to modify brackets etc.
Try the cars original middle section which you already should have cut down in the same place as the s4's exhaust, i did them at the same time. The middle section should mate up to exactly where is did when is was on the car previous. Once you know this is ok, i then cut 2 x sleeves out of 48mm out side diameter pipeing ( ruffly 6 inchs long each piece ) to act as sleeves to go into the original exhaust and the new s4 exhaust. I had hoped the s4 exhaust would of been a bigger bore size to my diesel but there is only 1mm difference so basically the same size pipeing hence the sleeve!!
I then welded around both ends of the sleeves to make a good seal and high strength joint, so no gas can escape.
Once all the exhaust is welded back into a full exhaust again, join up your front pipe with the clamp, make sure all your 4 x rubber mounts are on and have a look where the exhaust tail pipes are sitting.
You can use the adjustable bracket that acts as a support, to widen or close the rear boxes how ever you want it to sit. I also should mention i knocked the s4 back box mounting down so that they would sit up nice in the s4 rear valance.