For the 2018 Audi S4, both the low-beam and high-beam headlights use LED lighting. The supplied fitment data does not list a separate bulb number or identify projector and non-projector headlight variations, so verify the original headlamp assembly before ordering a replacement.
Other listed exterior lighting is also LED, including the front turn signals, daytime running lights, cornering lights, parking lights, front and rear side markers, tail lights, brake lights, center stop light, reverse lights, license-plate light, and rear turn signals. No fog-light bulb fitment is provided for this model year.
Interior lighting is predominantly LED as well, covering the dome, map, trunk, vanity, step, and general interior lights. The glove-box light is the notable exception and uses a 2821 bulb. Match the replacement to the specific location and confirm the existing bulb or lamp assembly when servicing an LED-equipped circuit.