In essence, everything except the military branches.
The term "Fourth Estate" — first popularized as a way to describe the press as a non-governmental actor in society — has in recent decades been appropriated as shorthand for the Defense Department's agencies and activities that are not part of the military branches: the Departments of the Army, Navy, and Air Force.
The distinction between these "DOD Components" and the "military components" is laid out in DoD Instruction 7730.64. As a 2018 GAO report put it, the "DOD has defined Fourth Estate organizations as DOD organizations, other than the military services, that have DOD manpower resources."
These organizations include the Office of the Secretary of Defense, the Joint Staff, DOD field activities, and the combatant commands. The list also includes more than two dozen defense agencies, including the Defense Logistics Agency, Defense Finance and Accounting Service, Defense Contract Management Agency, Defense Advanced Research Projects Agency, Missile Defense Agency, and more.
Fourth Estate field activities alone included more than 380,000 employees in 2018, according to the GAO's count.
In 2019, Defense Secretary Mark Esper directed Pentagon officials to scrutinize Fourth Estate spending in an effort to put money toward technology development and other priorities. In January 2020, he announced that he had found $5.7 billion to shift.