[Typ616]

2010 S4 here, CPO thank goodness (bought at 28K miles a year ago)... In the year + that I've had it, Transmission control module (mechatronics) @$4000, rear video camera (twice) with the line to it ($1500), MMI entertainment unit (not sure what bit), $$$, and I suspect it'll need control arms soon given the look of the bushings.

Friend with new 2013 S4 - Wheel bearings at 15K miles (!!!) and transmission - reprogrammed twice, replaced, also $$$$$ but under warranty.

So offhand I'd say no, not getting more reliable, but what a great car to drive under warranty !
I know mine goes for trade in the day before CPO warranty expires. And I'll get a stick and minimal electronics doodads, it should be more reliable specced that way (but on the S4, stick shifts miss out of the fabulous exhaust "farts" that DSG tranny forces the ECU to create)... The supercharged 3.0 engine is a beast, I love it ! If I can't get another S4 Ill get an A6 with that engine (if they come in stick). So much torque, on quattro, it would take something special to accelerate faster from 0->city speeds.