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.

Maltepe is a municipality and district of Istanbul Province, Turkey. Its area is 53 km2, and its population is 528,544 (2022). It is on the northern shore of the Sea of Marmara, adjacent to the Kadıköy district to the west, Kartal district to the east and Ataşehir district to the northwest. The mayor of Maltepe is Esin Köymen (CHP).