Caterpillar D4

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.

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 later D4 series engines quickly increased in power, so the number "4" just became a figure of merit rather than indicating actual engine power.

The Caterpillar D4 was used as the base tractor for the Trackson built loader that was known as the T4 or Traxcavator shovel.