The "RS" initials are taken from the German: RennSport—literally translated as "racing sport", and is the Audi marque's highest trim level, positioned above the "S" model specification of Audi's regular model line-up. Like all Audi "RS" cars, the RS4 pioneers some of Audi's latest advanced technology, and could therefore be described as a "halo vehicle".
https://en.wikipedia.org/wiki/Audi_RS_4