The 1939 Ohv Ariel Square Four - The 4G 955cc. The Aristocratic Brute - Smooth, Fast and Powerful. Legend has it that Edward Turner conceived the Square Four engine in 1928 - the essence of the concept appearing so quickly that he was required to write it down on the back of a cigarette packet, the engine was essentially a pair of 'across frame' OHC parallel twins joined by their geared central flywheels, with one four- block (or Monobloc) and one head, the idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the 'Ariel Square Four'.