What does the a in audi a4 stand for?

The "A" in the AudiA4 stands for "Audi". The numbers are used to denote different sizes of vehicle -- the A4 is larger than the A3, the A6 is larger than the A4, etc. It may sound a bit confusing, but if You think about it logically, it really is quite straightforward. Likewise, the S series stands for "Sport". They're also ordered from smallest to largest, from the S4 through the S8. Q denotes a Crossover, whereas R means "Roadster". RS stands for "RennSport" (or "Racing Sport"). Just remember that the higher the number, the bigger the car (and typically, the engine). If You're looking for a compact, You'll want to stay near the bottom of the range.