The 4 Lexington Avenue Express is a rapid transit service in the New York City Subway's A Division. Its route emblem, or "bullet", is colored green since it uses the IRT Lexington Avenue Line in Manhattan.
The 4 operates at all times. Daytime service operates between Woodlawn in the Bronx and Utica Avenue in Crown Heights, Brooklyn, making all stops along the IRT Jerome Avenue Line in the Bronx and express stops along the IRT Lexington Avenue Line in Manhattan and the IRT Eastern Parkway Line in Brooklyn; limited rush hour service is extended along the IRT New Lots Line to and from New Lots Avenue in East New York, Brooklyn. Night service serves all stops along its entire route except Hoyt Street and is extended to/from New Lots Avenue to replace the 3.
During the extension of the IRT Lexington Avenue Line north of 42nd Street–Grand Central Terminal, shuttle elevated trains served the IRT Jerome Avenue Line (which at the time was only between East 149th Street–Grand Concourse and Kingsbridge Road) starting June 2, 1917. On April 15, 1918, shuttles were extended to Woodlawn. A second shuttle, using subway cars, from 149th Street–Grand Concourse to Grand Central started on July 17, 1918.
On August 1, 1918, the entire Jerome and Lexington Avenue Lines were completed and the connection to the Broadway–Seventh Avenue Line at 42nd Street was removed. Trains began running between Woodlawn and Bowling Green, making local stops in the Bronx and express stops in Manhattan, except at night when they made all stops.
On August 23, 1920, the 4 was extended to and from Utica Avenue in Crown Heights, Brooklyn during daytime hours and New Lots Avenue in East New York during night hours with a few trains going there during rush hours. Trains ran express in Brooklyn during daytime hours but local during night hours.
From June 8, 2009, to June 26, 2009, New York City Transit conducted a pilot program for express service along the Jerome Avenue Line. Four morning weekday rush hour trains from Woodlawn stopped at Mosholu Parkway, Burnside Avenue and 149th Street–Grand Concourse before they resumed regular service in Manhattan and Brooklyn.
On October 26, 2009, another express pilot program on the Jerome Avenue Line was implemented based on the success of the first and ran until December 11, 2009. This program was the same as the one in June except that express trains stopped at Bedford Park Boulevard–Lehman College.
As a result of planned repairs to Hurricane Sandy-related damage in the Clark Street Tube, which carries the IRT Broadway–Seventh Avenue Line, between July 2017 and July 2018, the 4 will be extended to and from New Lots Avenue on weekends, making all stops in Brooklyn south of Atlantic Avenue–Barclays Center in place of the 3.