It's not one of the Big Four, but it is big. The Defense Finance and Accounting Service (DFAS) is essentially the payroll and accounting department for the Department of Defense (DoD). Annually, the DFAS pays more than 6 million military and civilian personnel, retirees, and annuitants; processes some 12 million commercial invoices; and makes about 8 million travel payments, disbursing about $578 billion, total. Additionally, it manages some $488 billion in military retirement and health benefits. DFAS is financed by its customers rather than through direct appropriations.
DFAS also provides assistance for electronic government projects for agencies outside the DoD, such as the Environmental Protection Agency, the Department of Energy, and the Department of Health and Human Services. Since its establishment in 1991, DFAS has reduced its footprint from more than 300 offices to nine sites and lowered its systems from about 330 to more than 110.