Why is it that I haven't heard about the possibility of upgrading the rear brakes with S4 parts?...
 

There're all sorts of BBK's and S4 swaps for the fronts; what about the rears?

Not needed. The brakes up front do most of the legwork.

overkill

I'm trying to understand why it's an overkill...
 

If they put those brakes in the S4, which more than anything else is more nose heavy than the A4, shouldn't the rear upgrade to S4 be less of an overkill than a front upgrade?

Sounds like you need to do a little learning...
 

In fact, it would adversely affect your braking by putting too much bias to the rear. When you decelerate, most of the weight shifts to the front, increasing pressure over the front contact patches. This is why there are usually larger rotors and multi-piston calipers in the front. Shifting this balance adversely could in fact increase stopping distances, or even worse, make your car more difficult to control under heavy braking.<ul><li><a href="http://www.stoptech.com/whitepapers/brakebiasandperformance.htm">Stoptech's Info on Brake Biasing</a></li></ul>

An upgrade should do one of three things (IMHO)....m....
 

1) Decrease your stopping distance.
2) Prevent brake fade after repeated stops.
3) Both 1 and 2.

(maybe 4, improve pedal feel, but that's a bit of a stretch to me)

Switching from A4 rears to S4 rears (with nothing else) will do none of the above, hence, I see no use for it. There are many other brake improvements that can be done to do 1,2, or 3, that don't involve any changes in back.

Thanks for the link, though I actually have read that before...
 

And it's also my understanding that rotor size doesn't have a huge effect on braking power compared to pads. That being said, wouldn't a larger disc at the rear help with keeping temperatures down, especially if (and this is an if I'm still trying to have somebody tell me) the rears are vented, and thus a "worthwhile" upgrade?

read below. Better off spending that money on some Azian massage

Pardon me for not being clear, but I didn't mean upgrading just the rears...
 

I mean, couldn't the rears be upgraded as the fronts are?

Just something that popped into my head in a boring afternoon.

However, if you're upgrading the front brakes you should do something to maintain brake bias