At a meeting in Paris in late 1949, the sporting commission of the FIA decided to recognise the 500cc National Formula as the basis of  new International Formula 3 for 1950. Cars must have a minimum ground clearance of 4 inches and a minimum weight of 440 lbs, dry. International F3 races must be run on circuits of at least 1,600 yards per lap and exceed 30 miles in total.

Cooper cars had a full order book so the Mk IV, introduced for 1950, represents an evolution of the Mk III car. The ladder frame chassis, with hoops to support the body, was retained and the principle objective was to save a little weight. Longer Hardy Spicer drive shafts were introduced and Lockheed twin leading shoe brakes became standard at the front and rear. Twin master cylinders allowed a fully split baking system to be introduced. Cars were built with 500cc JAP engines (T11) and a long chassis version (T12).

J. A. Prestwich had been impressed with Cooper's record and specially developed an 1,100 twin with a dry sump, modified heads, aluminium barrels and a radical camshaft. Power output, on Methanol was about 95 bhp at 6,000 revs. Officially, the car was only offered with a 500cc JAP engine due to the difficulty of obtaining Norton "double knocker" units but cars were supplied without engine for customers to source their own. Sadly, the Grand Prix driver and Le Mans winner, Raymond Sommer was killed in a crash in his T12 at Cahours in September 1950.

Advert from Motor Sport March 1952

John Gales' long chassis car (T12)

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In late1957, a modified Mk IV, built by Arthur Owen, with a streamlined glass fibre body and 250cc Norton engine was used by Bill Knight to set five records at Monza.