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As men became available on Corregidor from January until after the fall of Bataan, they were integrated into the 4th Marines to support beach defense. In February, 58 sailors formerly of the USS Canopus were organized as a reserve company Lieutenant Clarence Van Ray with Platoon Sergeant Leslie D. Sawyer and Sergeant Ray K. Cohen trained and equipped the sailors into an efficient fighting force. Ten Marines and 40 more sailors were added to the company after the fall of Bataan.

The largest group of reinforcements arrived after the fall of Bataan. In the days following 9 April, 72 officers and 1,173 enlisted men from more than 50 different organizations were assigned to the 4th Marines, making the Marine regiment one of the most unusual units in Marine Corps history. These reinforcements included members of the Navy, the Army, the Philippine Army and Philippine Scouts. Sailors stranded on land after the loss of their ships found themselves alongside engineers, tankers, and aviators whose units were captured on Bataan. Filipino Scouts were assigned with members of the islands' Constabulary to the 4th Marines. Unfortunately, very few of the reinforcements were trained or equipped for ground combat. By 29 April, the 4th Marines numbered 229 officers and 3,770 men, of whom only about 1,500 were Marines.