The 6R4 (6 cylinder, rear engine, four-wheel-drive) was developed by Williams Grand Prix Engineering and built to comply with the international Group B regulations for rally cars in the 1980s. It is fitted with a purpose-built V6 four overhead cam 24-valve engine. Mid-mounted, it drives all four wheels via a five-speed gearbox and a Ferguson-Formula central differential.

200 6R4 Clubman cars were built to allow homologation, and even in Clubman guise with a modest 250 bhp, 60 mph could be reached in 4.3 seconds.

The 6R4 appeared in February 1984 and scored its first victory at the Gwynedd Rally of March 1985 but sadly and shortly afterwards, Group B regulations were changed to abolish the class for these exotic machines.