Edward Turner conceived the Square Four engine in 1928. Turner would later go on to design the Triumph twin and become the the head of the Triumph Company, but at this time he was looking for work, taking drawings of his revolutionary engine around the motorcycle industry. The idea for the engine, which was comprised of a pair of 'across frame' parallel twins linked by a pair of gears, was adopted by Ariel, then very much a force in the British motorcycle industry. The engine had its problems but in theory was almost a perfect motor, compact and well balanced.

The prototype was a 500 cc capacity engine, so well contained that it fit into the frame of the Ariel 250. The basic design was worked on and a modified version was released in 1930. It caused a massive stir, and as it was ideal for sidecar riders, an upgraded 600 cc version was released in 1932 specifically for this purpose.

In 1937 the Square Four went through ac complete redesign. The old engine had been fitted with an overhead camshaft that had led to the engine's major weakness - a tendency to overheat around the cylinder head. The 1937 engine replaced the camshafts with pushrods and the crank and crankcase were completely changed. A 1000 cc option was added to the standard 600 cc for the first time. Production ceased during the war and afterwards only the 1000 cc model was built and offered with telescopic forks. In 1949 the engine became all alloy and the overall handling and acceleration improved.

The Square Four 4G MK II in the Mathews Collection was built between 1953 and 1958. It had multiple engine upgrades from the original and other changes include four separate exhaust pipes. It was an incredible machine, the Square Four engine combined with the expansive 1000 cc capacity, that had by far the largest British engine available at the time.