Fourth Echelon (also known as 4th Echelon, abbreviated as 4E) is a "blacker than black" clandestine special operations and counter-terrorism unit that answers solely to the President of the United States. Commissioned by President Patricia Caldwell, the unit is mandated to deal with terrorist threats to U.S. interests.

The unit's commander is former Third Echelon Splinter Cell Sam Fisher, who was instrumental in foiling an assassination attempt on President Caldwell's life orchestrated by Third Echelon director Tom Reed. Former Third Echelon chief technical advisor Anna Grímsdóttir, who previously worked closely with Fisher when he was with Third Echelon, serves as the unit's technical operations manager. Adding to the unit is Sam's war buddy Victor Coste, former CIA operative Isaac Briggs, and computer hacker Charlie Cole, while Andriy Kobin become a source of intelligence inside holding cell.

The unit has no permanent ground base. It operates out of the Paladin, a large military transport aircraft repurposed as a stealth airliner for their mobile HQ.

The unit's first and present assignment is the disruption of a planned terrorist action of escalating attacks against US interests known as the Blacklist which masterminded by the group of 12 terrorist called The Engineers, with shutting down the elements of Third Echelon being a secondary objective.