Its route emblem, or "bullet", is colored forest green since it uses the IRT Lexington Avenue Line in Manhattan.

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.

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.

[9] On November 17, 1924, the Transit Commission ordered the IRT to use 100 new subway cars to increase service by no later than December 1.

[11] The Transit Commission heeded the IRT's recommendation not to have half of 4 trains run express due to the dangerous operating condition it would have created.

Instead, the Transit Commission accepted the IRT's plan to allocate 70 new cars to West Side express service to Brooklyn.

[12] Express service along Eastern Parkway would start at the earliest in February 1925 when additional new cars arrived.

[20] As of 1934, 4 trains ran from Woodlawn to Utica Avenue weekday rush and Saturday morning peak and afternoon; to Atlantic Avenue weekday midday, Saturday morning after the peak, and late nights; and to South Ferry evenings and Sundays.

[citation needed] Beginning on May 10, 1946, all 4 trains were made express during late nights running on twelve-minute headways as the 6 went back to Brooklyn Bridge during that time.

[citation needed] Starting on December 15, 1950, four 4 trains began operating during rush hours in the peak direction to and from Flatbush Avenue on the Nostrand Avenue Line, with the four trains in the AM rush hour leaving every 16 minutes between 7:59 and 8:47 a.m., and the four trains in the PM rush hour arriving every 16–20 minutes between 5:20 and 6:13 p.m.[25] Also on that day, weekday midday service was cut back from Atlantic Avenue to South Ferry.

[26] On March 19, 1954, late-night service in Brooklyn became local, but resumed operating express between Atlantic and Franklin Avenues on June 29, 1956.

[27] As a result of the opening of the main portion of the Chrystie Street Connection along the Manhattan Bridge on November 26, 1967, the 4 train was color-coded magenta under the first color scheme.

By 1972, the 4 began to skip 138th Street weekdays during rush hours in the peak direction which it continues to do (mornings to Manhattan and evenings from there).

[7][30] This service cut affected 15,000 riders and was criticized by Manhattan Borough President Andrew Stein as no public hearing was held.

[34] Late night express service was reinstated from January 21, 1990, to October 5 of that year, as a result of the 6 being extended back to Brooklyn Bridge during that time.

[35] In January 1991, a reduction of service along the Eastern Parkway corridor to remove excess capacity was proposed.

[41] As a result of planned repairs to Hurricane Sandy-related damage in the Clark Street Tube, which carries the IRT Broadway–Seventh Avenue Line, the 4 was extended to New Lots Avenue on weekends from June 17, 2017, to June 24, 2018, making local stops in Brooklyn south of Nevins Street in place of the 3.

[42][43] On November 17, 2019, New York City Transit made adjustments to weekday evening 3, 4, and 5 service in order to accommodate planned subway work.

[3] PATH at World Trade Center Staten Island Ferry at Whitehall TerminalSouthern terminus of two a.m. rush hour trips and special event express service