The Denmark Transport Ministry has inaugurated the southern extension M4 metro line to Sydhavn and Valby in Copenhagen, Denmark.

Five metro stations are served by the 4.5 km long southern extension, which also serves as a center for connections with regional and national trains as well as the S-train network. About 75% of residents in the Copenhagen and Frederiksberg neighborhoods will have quick access to a train or metro station less than 600 m. Hitachi Rail created a digital signaling system that combines communication-based train control (CBTC) and driverless technologies. Trains can operate safely at shorter intervals thanks to CBTC technology's ability to boost service capacity and decrease the demand for physical roadside equipment. After the successful completion of M1 and M2, which opened in 2002, and M3 (Cityringen), which was inaugurated in September 2019, M4 is the fourth metro project in Copenhagen by Hitachi Rail.

Five additional stations are part of the M4 line extension: Mozarts Plads, Havneholmen, Enghave Brygge, Sluseholmen, and København Syd. By linking Orientkaj in Nordhavn with København Syd in Valby, this extension improves the metro system's overall performance and offers commuters more convenience. There are currently 44 stations and 43 km of track in the metro network. Passenger traffic on the M3/M4 lines is anticipated to increase dramatically as a result of the M4 extension. More specifically, it is anticipated that the five additional stations will serve about eight million people a year by 2030.