On July 17, 1918, the Lexington Avenue Line local tracks were opened, allowing another shuttle service to run between 149th Street–Grand Concourse and Grand Central. On April 15, 1918, with the extension of the Jerome Avenue Line to Woodlawn, shuttle service was also extended. Since the extension of the IRT Lexington Avenue Line between 149th Street and Grand Central was not yet open, this section was served by shuttle trains using elevated train cars. Service on what was later known as the 4 began on June 2, 1917, as the first portion of the IRT Jerome Avenue Line opened between 149th Street-Grand Concourse and Kingsbridge Road. Until 1983, rush hour 4 trains originated and terminated at Flatbush Avenue–Brooklyn College in Brooklyn. For up to an hour after evening events that are held at Yankee Stadium, a special downtown-only express service runs between 161st Street–Yankee Stadium and Bowling Green. Late night service makes all stops except for Hoyt Street. During rush hours in the peak direction, 4 trains skip 138th Street–Grand Concourse. Daytime service operates between Woodlawn in the Bronx and Utica Avenue in Crown Heights, Brooklyn, via the IRT Jerome Avenue Line in the Bronx, express via the Lexington Avenue Line in Manhattan, and the IRT Eastern Parkway Line in Brooklyn limited rush hour service, as well as late night service, is extended beyond Utica Avenue to/from New Lots Avenue in East New York, Brooklyn via the IRT New Lots Line. Its route emblem, or 'bullet', is colored forest green since it uses the IRT Lexington Avenue Line in Manhattan.

The 4 Lexington Avenue Express is a rapid transit service in the A Division of the New York City Subway.

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