The diesel powered Caterpillar D4 evolved from the Cat R4 gasoline tractor. In 1936, the Cat R4 was joined by the diesel RD4 (4G series) and in 1937 the RD4, with some minor modifications, was renamed the D4 (4G series).
The early D4 models (4G, 7J, 2T, 5T) were powered by the Cat D4400 engine developing 35 drawbar horsepower. A small gasoline pony motor was used to start the diesel. The 6U and 7U were upgraded to the Cat D315 4 cyl diesel with 43 drawbar hp.
Caterpillar D4 tractor mired in deep mud during Alcan Highway construction, Alaska, 1942.
During WW II, the D4 was one of the tractor/bulldozers collectively known as "Tractor, Light." The Caterpillar D4 models were Ordnance G-151. Equivalent tractors were produced during WW II by International Harvester Co. (Model TD9, G-99). The Cat D4 was used as a prime mover for light artillery, for general duty as a tractor, or as a bulldozer when equipped with a suitable blade attachment.
The evolution of the D4 included these models:
D4 4G series 1937-1938, Orchard model
D4 7J series 1939-1942
D4 2T series 1943-1945
D4 5T series 1945-1947
D4 6U series 1947-1959, 44 inch gauge
D4 7U series 1947-1959, 60 inch gauge
Over 7,400 of the 5T series were built and over 44,000 7U units were produced. Tractor serial numbers were composed of the series (eg 4G, 6J) followed by a four or five digit serial number. With modifications and additional series, the D4 continued in production and remains in the Caterpillar product line in the mid-2000s.
Thanks to Stephen A. Sarkany for help with this page.
Caterpillar D4 tractor in use during Alcan Highway construction, 1942.