After the 1963 season, Sunako made the switch to four-wheel racing and signed with the Prince Motor Company as a works driver.

He drove his Prince Skyline S54 to a second-place finish in the GT-II sports car race, behind the Porsche 904 of Sokichi Shikiba.

[1] In 1966, Sunako won the Japanese Grand Prix sports car race at Fuji Speedway in his Prince R380.

He finished the year as the champion of the All-Japan Racing Driver Championship in the touring car division.

Sunako did not drive during the 120 lap race, but is officially credited as a winning driver alongside Kurosawa.