The Audi S4 can be obtained through the "Steal a Parked Car" and "Hijack a Car" Grand Theft Auto crimes. It is obtained about 5% of the time (or roughly one time for every 20 successes) from "Hijack a Car." It can only be obtained from "Steal a Parked Car" when a player's Nerve bar is 40 or less. 
The Audi S4 cannot be found through city finds but can be traded between players.
- Damaster, GTA Crime Stats (14/02/23)