Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Caterpillar D4 track-type tractor is a small bulldozer manufactured by Caterpillar Inc. Caterpillar first introduced the RD4 in 1938 as the diesel follow on to the successful CAT 30 gas model. The RD4 had a D4400 engine. In 1935 Caterpillar started the naming convention of "RD" for diesel or "R" for regular gasoline followed by a number to indicate the relative size. This was later changed to just a D for the diesel versions. At the time of introduction the D4 engine produced about 43HP at the drawbar so the 4 indicated the relative engine power. The D4 U series was fitted with the new D315 engine. The D4 series engines quickly increased in power so the numbers just became a figure of merit rather than indicating actual engine power.