B4 Series Rear Driver or Passenger Side Air Monotube Shock Absorber (26-269993) by Bilstein®. Quantity: 1 per Pack. The Bilstein B4 air suspension module. The only fully functional module for all air suspension systems in uncompromising OE quality. High-quality air suspension modules in uncompromising original manufacturer quality have only been available for a few years. Unfortunately this is why in the meantime, spare parts of significantly poorer quality and often with considerably restricted functions have spread on the market.