The fourth season of the television series The Wire commenced airing in the United States on September 10, 2006, concluded on December 10, 2006, and contained 13 episodes. It introduces Baltimore's school system and several middle school students while continuing to examine the remnants of the Barksdale Organization, the ascendant Stanfield Organization, the Baltimore Police Department and politicians.

The fourth season aired on Sundays at 10:00 pm ET in the United States. The season was released on DVD as a four-disc boxed set under the title of The Wire: The Complete Fourth Season on December 4, 2007 by HBO Video.

Playwright and television writer/producer Eric Overmyer joined the crew for the show's fourth season as a consulting producer and writer. He had previously worked on Homicide and was brought into the full-time production staff to replace George Pelecanos who scaled back his involvement in order to concentrate on his next book and he worked on the fourth season solely as a writer. David Mills, Emmy-award winner, Homicide and The Corner writer and college friend of Simon, joined the writing staff in the fourth season. Regular writer Ed Burns became a producer on The Wire in this season.

The focus of the fourth season shifted between a local school, the mayoral election, police department politics and action on the street corners. The returning starring cast consisted of Dominic West as Officer Jimmy McNulty, the formerly insubordinate detective who attempts to shed his ability and his problems in favor of a better life. West was at the time homesick and wanting to spend time with his young daughter in England; he also felt the character's plot arc had reached a reasonable end point in season 3, so West arranged with the writers for McNulty's role to be greatly reduced in Season 4.

Lance Reddick reprised his role as newly promoted Major Cedric Daniels, now commanding the western district. One of Daniels's sergeants within the district was Sergeant Ellis Carver, portrayed by Seth Gilliam. Robert Wisdom reprised his role as former western district commander Howard "Bunny" Colvin, who has become a field researcher after a short stint working in hotel security following his retirement from the Baltimore Police Department.

The Major Crimes Unit saw a shift in personnel this season. Kima Greggs, portrayed by Sonja Sohn, and Lester Freamon, portrayed by Clarke Peters, transferred to the Homicide Unit after the new lieutenant of the Major Crimes Unit ends the wire and halts Freamon's investigation of the Barksdale money. Corey Parker-Robinson portrayed Detective Leander Sydnor, one of two detectives who remain in the Major Crimes Unit after the arrival of the new lieutenant. Domenick Lombardozzi returned as Thomas "Herc" Hauk, a former member of the Major Crimes Unit whose work on the mayor's security detail earns him a promotion to sergeant and a transfer back to his old unit after Freamon and Greggs' departure.

Wendell Pierce portrayed veteran homicide detective Bunk Moreland. Deirdre Lovejoy starred as assistant state's attorney Rhonda Pearlman, the legal liaison between the unit and the courthouse. Andre Royo returned as Bubbles, who continued to indulge his drug addiction and act as an occasional informant. Jim True-Frost portrayed Roland "Prez" Pryzbylewski, a former member of the Major Crimes Unit who has become a teacher in an inner city school after inadvertently killing a fellow officer in season three.

The police were overseen by two commanding officers more concerned with politics and their own careers than the case, Deputy Commissioner of Operations William Rawls (John Doman) and Commissioner Ervin Burrell (Frankie Faison). At city hall, Tommy Carcetti (Aidan Gillen) was an ambitious city councilman seeking to become mayor. Joining the cast for the fourth season was Reg E. Cathey as Carcetti's deputy campaign manager, Norman Wilson. Also joining the cast after having a recurring role during the third season was Glynn Turman as Mayor Clarence Royce.