Line M4 links Kadiköy station with Kartal and runs underground for its entire 21.8km length. The line is equipped with Thales Seltrac CBTC and has a design headway of 90 seconds with a service headway of two minutes, giving design capacity of 70,000 passengers/hour/direction. Services are operated by a fleet of 30 six-car trains supplied by CAF. The journey time between the termini is 32 minutes.

Istanbul Metropolitan Municipality (IMM) says the line will initially carry around 700,000 passengers per day.

The total cost of the project was €751.3m. A 3.4km, €150.2m extension to Kaynarca is currently under construction.