The M4 was based on the chassis and drive train of the obsolescent M2 Light Tank which introduced the trailing idler. This common practice of re-using old vehicles simplified design, allowed for easy production, and made maintenance in the field easier.
One variant was designed to tow anti-aircraft guns and another for howitzers. The rear compartment carried the gun crew and other equipment and some later variants included a crane to assist with heavier projectiles.
Two types of ammunition boxes were used o