APR has a stainless steel, mandrel bent, cat-back exhaust for   the S4.

APR's S4 Exhaust is a stainless steel, mandrel bent, cat-back   exhaust. Our exhaust begins after the two lower factory catalytic converters and   is two 2.25" pipes that merge into one 3" pipe. This exhaust is very quiet and   simply changes the tone of the exhaust to a deep, unobtrusive sound. With TIG   welding, fully stainless construction, and CNC'ed hangers, you will not find a   higher quality system on the market.

APR's S4 cat-back exhaust is available with two different   sound levels. The Stealth exhaust helps to maintain the S4's subdued character.   While louder than stock, it is not too loud to attract attention. The SRT   exhaust replaces the center muffler with a special side resonator. This exhaust   has a more aggressive sound and a distinctive note.