The Fourth Army was authorized by the National Defense Act of 1920 and was originally to be composed of Organized Reserve units primarily from the First, Second, and Third Corps Areas.

The Fourth Army mission was to develop defense and operational plans for contingencies in the vicinity of the Pacific coast and the western United States, review the mobilization plans of the Seventh and Ninth Corps Areas, and oversee the training of units in the army area.

As part of the responsibility to oversee training, the Fourth Army planned, conducted, and/or participated in three major maneuvers between 1937 and 1941.

The 1937 maneuvers, under the command of Major General George S. Simonds, were actually four separate exercises and were held at Fort Riley, Kansas, and Camp Ripley, Minnesota, for the Seventh Corps Area troops, and at Camp San Luis Obispo, California, and Fort Lewis, Washington, for the Ninth Corps Area troops.

The next Fourth Army maneuver was also a split exercise, this time under the command of Lieutenant General John L. DeWitt.

Following the maneuvers, the Fourth Army headquarters returned to its station at the Presidio of San Francisco, where it was located on 7 December 1941.