The R4 layout is very rare, only having been used on a small number of production vehicles. Notable vehicles with this layout include several high-performance Porschesports cars, including the 959, the 911 Turbo since the introduction of the turbocharged version of the 993 series in 1995, and the 911 Carrera 4 introduced with the 964 series in 1989.
Ludvigsen, Karl (2003). Porsche. Excellence Was Expected. Vol.3. Cambridge, Mass.: Bentley Publishers. ISBN0-8376-0235-1.[pageneeded]
Taylor, Blaine (2004). Volkswagen Military Vehicles of the Third Reich. Cambridge, Massachusetts: Da Capo Press. ISBN0-306-81313-0. OCLC55534990.[pageneeded]