Seriously though, as Dummi has said, the S4 uses metered air. In other words the ECU reads how much air is coming into the system via the MAF and expects it to stay in there!
If you fit dump valves, the ECU still thinks the metered air is in the system, so when you put your foot down, you'll be running rich. It'll only run rich very briefly, but over time this could potentially cause problems.
I've seen some american guys have them fitted, and some companies sell them specifically for the S4 (turbosmart I think), but I personally wouldn't bother.