Compare the Audi A8 4.2 TDi Quattro - [2013] vs Audi A4 S4 4.2 Quattro Cabriolet - [2004]. Find out which car is the fastest, most powerful, lightest or has the biggest engine in our top trumps style comparison page. Find out which car has the highest top speed or is the fastest accelerating, is it the Audi A8 4.2 TDi Quattro or the Audi A4 S4 4.2 Quattro Cabriolet. We'll leave it up to you to decide which is your favourite.