The term 'COPTER's Forerunner' can be solved by the four-letter word 'Giro'. Giro is a type of aircraft that was developed in the early 20th century as an alternative to the helicopter. It was designed to be more efficient and maneuverable than the helicopter, and it was the first successful rotary-wing aircraft. Giro aircraft were used in World War I and II, and they paved the way for the development of the modern helicopter.