Caterpillar first introduced the D4 in 1938 as the diesel follow on to the successful CAT 30 gas model. In 1935 Caterpillar started the naming convention of 'D' for diesel or 'R' for regular gas followed by a number to indicate the relative size. At the time of introduction the D4 engine produced about 40HP so the 4 indicated the relative engine power. The D4 series engines quickly increased in power so the numbers just became a figure of merit rather than indicating actual engine power.