Caterpillar first introduced the RD4 in 1936,[1] as the diesel follow on to the successful CAT 30 gas model. The RD4 originally weighed in at 10,000 pounds (4,500 kg), and used Caterpillar's D4400 engine, an inline four-cylinders, with a 4.25 by 5.50 inches (108 mm × 140 mm) bore and stroke.[2] In 1935 Caterpillar had started the naming convention of "RD" for diesel or "R" for regular gasoline, followed by a number to indicate the relative engine size.[3] In 1937, the "R" was dropped, and just a "D" was used for the diesel versions. At the time of introduction of the RD4, the D4400 engine produced about 43 horsepower (32 kW) at the drawbar so the 4 indicated the relative engine power. The D4 U series was fitted with the more powerful D315 engine. The later D4 series engines quickly increased in power, so the number "4" just became a figure of merit rather than indicating actual engine power.
