The M4, officially referred to as the M4 Kadıköy - Sabiha Gökçen Airport metro line, is a 33.5-kilometre (20.8 mi), 23-station rapid transit line of the Istanbul Metro. It is colored deep pink on the maps and route signs. Running between Kadıköy and Sabiha Gökçen Airport, it is the first rapid transit line operating on the Asian side of Istanbul. The M4 mostly runs under State road D100, parallel to the Istanbul-Ankara railway and is entirely underground.

The Istanbul Metro is a rapid transit railway network that serves the city of Istanbul, Turkey. Apart from the M11 line, which is operated by TCDD Taşımacılık, the system is operated by Metro Istanbul, a public enterprise controlled by the Istanbul Metropolitan Municipality. The oldest section of the metro is the M1 line, which opened in 1989. As of March 2024, the system now includes 158 stations in service, with 36 more under construction. Istanbul is the city with the 22nd longest metro line in the world and the 4th in Europe.