M4 (Istanbul Metro)

The M4 line is a rapid transit line of the Istanbul Metro system. Officially named the M4 Kadıköy–Sabiha Gökçen Airport line, it operates on a north-south axis entirely within the Asian side of Istanbul. The line connects the Kadıköy transportation hub on the coast of the Sea of Marmara with the Sabiha Gökçen International Airport.

Opened in 2012, the M4 line initially ran between Kadıköy and Kartal. Subsequent extensions have expanded the line further east, culminating in its current endpoint at Sabiha Gökçen Airport. The line is primarily underground, utilizing bored tunnels.

The M4 line is color-coded pink on Istanbul Metro maps. It provides important connections to other modes of transportation, including ferries at Kadıköy and buses throughout the region. It is a vital part of Istanbul's public transportation network, facilitating access to the airport and connecting various residential and commercial areas on the Asian side of the city. The operator of the line is Metro Istanbul, a subsidiary of the Istanbul Metropolitan Municipality. The line's construction significantly improved accessibility to Sabiha Gökçen International Airport from central Istanbul.