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 of the Department of Defense (DoD). Annually, the DFAS pays more than 6 million military and civilian personnel, retirees, and annuitants; processes some 11 million commercial invoices; and makes about 7 million travel payments, disbursing about $557 billion, total. Additionally, it manages some $631 billion in military retirement and health benefits. DFAS is financed by its customers rather than through direct appropriations. It also provides assistance for electronic government projects for other agencies, such as the EPA and the Department of Health and Human Services.
DFAS was established to standardize, consolidate, and improve DoD's financial and accounting services. It has reduced its footprint from more than 300 offices to nine sites and reduced its systems from more than 300 to about 110.