I had a 2000 Audi A4 1.8T and a 2003 Audi A4 1.8T. I now drive a 2007 Lexus IS250. Simply put, both are GREAT cars, but seeing how this is the Lexus forum, well, you're gonna see lots of Lexus love. Similarly, Audiworld posters will never tell you to go with Lexus.
What I've seen... and I live in Miami so little need for 4WD here anyway...
My IS is perfect tech-wise thus far, zero unscheduled service visits, it's built quite flawlessly. My A4s were also reliable, but once or twice I did have unscheduled issues that were minor, but hey, they happened. European cars have their "temper" ;). Nothing that would keep me from going with Audi again, I was just not loving the new design of the A4 too much, but probably just bored of Audi.
I'd repeat what the poster above says, it comes down to you. Despite what people say either way, both the IS and the A4 are excellent vehicles and I'd have zero problem driving either one. I did get a much better deal financially with Lexus than Audi, but that's mostly due to resale value, etc etc.