I agree the TDI is a safe bet as long as it has been maintained and serviced correctly the car will do many miles..
The Audi is pretty much like any other car, if its not looked after and driven hard all its life expect to get some problems..
In my opinion if you really want a sporty A4 take your time in finding one wether it be petrol or diesel look for FSH and low amount of owners, mileage not too important as these cars are good for big miles..
I bought my wifes A3 1.8T sport off Ebay for £3500 2 years ago now with 142k on the clock with full Audi service history and all we have had to replace is the wiper motor !!!
All running gear ie engine, gearbox and turbo are all original so dont be put off with higher mileage examples !!
Good luck with your search for a good one and let us kow how you get on
Cheers T-sport