4-F is a Selective Service category that indicates an applicant has been deemed unfit for military service. Military Entrance Processing Stations make this determination based on the necessary physical, mental and moral standards required of members in military service.
The Selective Service Act of 1917 granted the President of the United States the power to conscript men for military service. The modern draft system was created by the Selective Service Act of 1948. Every male U.S. citizens is required to register with the Selective Service within 30 days of his 18th birthday. Each man is assigned a classification, including classifications for unrestricted military service, student deferment, conscientious objector and unfit for duty.