So in addition to my S60 and V70, I recently took ownership of my father's Audi S4. Brakes were shot so I ended up overhauling the stoppers. While I had the calipers off, I noticed that the ATE single-piston calipers bore a striking resemblance to the XC90 calipers (336mm) I upgraded to on my V70.

I had a spare set of XC90 calipers and after experimenting, I found that the 336mm XC90 calipers' bolt right up. Dimensionally they are the same but with one key difference: the mounting holes are spaced apprx 2mm wider for the S4 carrier. I used the guide pins from the S4 carrier as they're qual length while the XC90's are of unequal length (the aft pin is longer).

Also, while the S4 and XC90 pads are appear nearly identicaly, the mounting clips/prongs on the pad backing plates have ever-so-slightly different locations such that they cannot be used interchangeably because they wont seat properly (ended up using S4 pads, cutting off the sensor wires).

Coincidentally, I had spare pair of S4 rotors I measured against the XC90 rotors. Same 68.0mm center bore, with a larger hat and different bolt pattern. I just clamped them up together so i could use the XC90's bolt pattern as a guide and drilled the S4 top hat, worked like a charm (if anyone questions the safety of this, I've drilled top hats numerous times with zero issues).

With some steel washers as spacers, the S4 calipers mounted perfectly over the rotor. I was already using 10mm spacers so the calipers just barely cleared. After a few weeks and hundreds of miles of driving to bed them in, I can honestly say these are a noticeable improvement over even the S60R setup I have on my T5. I've attached a few pics and will upload more.

 

So a few folks messaged me asking about fitment and how they appear under the rims. These are 17" Volans and the front calipers have about 3mm clearance.

I'm still running the S60R calipers in the rear and havent noticed any brake balance issues so far. Also, the weight difference between the S4 and S60R calipers is quite negligable so no noticeable adverse affect on the handling.

Some folks inquired why I switched out the S60r calipers and its pretty simple: I can easily get rebuilt calipers from Autozone for $100 a pair (minus core charge) and Audi S5 rotors from the same so cost was the biggest driver.