One of the four original numbered air forces, Fourth Air Force was activated as the Southwest Air District of the GHQ Air Force on 18 December 1940, at March Field, California. It was redesignated Fourth Air Force on March 26, 1941 with a mission for the defense of the Southwest and Lower Midwest regions of the United States.
During World War II, Fourth Air Force was the primary air defense command for the West Coast. The command also flew antisubmarine patrols along coastal areas of the Gulf of Mexico from after Pearl Harbor until October 1942. One of its primary fighter units was the 10th Fighter Wing at Hamilton Field, California.
In September 1942, Rice Municipal Airport located in the Desert Training Center was acquired by the IV Air Support Command, and was operational by October, 1942. Re-designated Rice AAF it was used to train pilots and crews of aircraft whose mission it was to support ground troops.
Beginning in May 1942, the mission of Fourth Air Force became operational training of units and crews, and the replacement training of individuals for bombardment, fighter, and reconnaissance operations. It received graduates of Army Air Forces Training Command flight schools; navigator training; flexible gunnery schools and various technical schools, organized them into newly activated combat groups and squadrons, and provided operational unit training (OTU) and replacement training (RTU) to prepare groups and replacements for deployment overseas to combat theaters. The Fourth Air Force became predominantly a fighter OTU and RTU organization. Most P-51 Mustang and P-38 Lightning groups were trained by Fourth Air Force primarily due to the proximity of their manufacturing plants in Southern California. By 1944, most of the Operational Training of groups ended, with the command concentrating on RTU training of individual replacements using Army Air Force Base Units (AAFBU) as training organizations at the airfields controlled by Fourth Air Force.
Air Defense Wings were also organized for the major metropolitan areas along the West Coast, using training units attached to the Wings. By 1944 the likelihood of a full-scale air attack along the West Coast since the bombing of Dutch Harbor two years earlier was remote, and these air defense wings were reduced to paper units.
In December 1944, First, Second, Third and Fourth Air Force were all were placed under the unified command of the Continental Air Forces.