It finally died after 4 and 3/4 years (including 3 winters in upstate New York). I searched on the forum and decided i would go with a conventional battery as opposed to the lightweight battery units some guys are going towards. I just didn't have the time to fab up a nice way to hold the battery in place, which I would have wanted.

Anyway, I got the Autozone 48-DL Duralast battery. I can confirm that it fits like a glove in my 2000 S4 without any adjustment. I didn't have a scale so I can't compare weights or anything. It comes with a vent tube kit that mates to the stock tub perfectly. I have to say I'm very happy with it, especially considering it only cost $65. Of course the real test comes four or five years from now when it's freezing cold out, but odds are I wont be the one testing it at that point. Those of you looking to save a few bucks and don't mind installing it yourselves might want to go this route.

Update: this battery may not fit all years of the S4. It fits perfect in my 2000 S4. If you have the battery hold-down on the left in the pic, it should fit perfectly

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*I am not a mechanic nor an expert in Audi repair/tuning. In no way are my posts meant to replace professional consultation.*