And here's a PM I sent someone on Vortex regarding the S4 bumper:
To help you out.
I have looked for a long time and that's the best deal I found.
I'm not sure how much you've looked into getting a S4 front bumper, but trust me, it won't be cheap. I got lucky and found someone local who didn't know what he was sitting on, so I picked mine up with almost everything (filler plate, lower center grille, right side fog grille, washer caps and tow hook cover) for $200 and had it painted.
On average, I see these things sell on Audizine for $350-400 usually missing a few components and barely ever in the color I needed. The only time I found a complete bumper in silver, it was $500.
The link above I think is a completely bare bumper, meaning you'll need the washer caps and tow hook cover. I know the washer caps from your A4 bumper won't work on the S4, but the tow hook cover might. You'll also need the fog light grilles and lower center grille along with the washer jet guides - the jet guides are the plastic housing in which your headlight sprayer sits in.
Here's a list of part numbers that you may need. http://genuineaudiparts.com/
is the cheapest place to find these parts unless you know someone at a dealership and can get dealer pricing. Dealer pricing is pretty much the same, you just save on not needing to ship.
Fog light left side - 8E0941699A
Fog light right side - 8E0941700A
Fog grille left side - 8E0807681B01C
Fog grille right side - 8E0807682B01C
Fog grille mounting kit (this part is needed ONLY if the bumper is missing the insert tab for the fog grille screw to lock into. Otherwise the fog grille already comes with the screw meaning you won't need this.) - 8E0898193A
Headlight washer cap left side - 8E0955275BGRU
Headlight washer cap right side - 8E0955276BGRU
Lower center grille (black surround) - 8E0807647F5PV
Lower center grille (chrome surround) - 8E0807647B3SR
Washer jet guide left side - 8E0807787A
Washer jet guide right side - 8E0807788A
Plate filler - 8E0807977GRU
Things to note:
* Your fog lights will not work with this bumper - the mounting brackets are different.
* Your washer caps will not work as the shape is different.
* Your tow hook cover may work, but I have not tried.
* It's difficult to find a complete bumper, but if you want one it's worth looking around in the classifieds and hoping you can find one.
* It's not going to be a cheap swap, unless you get really lucky.