Metro Line M4 (Budapest Metro)

Line 4 (officially: South Buda–Rákospalota (DBR) Line, Metro 4 or M4, and unofficially: Green Line) is the fourth line of the Budapest Metro. It opened on 28 March 2014.

The first section, 7.4 km (4.6 mi) in length and consisting of ten stations, connects the southwestern Kelenföld vasútállomás located in Buda, and the eastern Keleti pályaudvar in Pest, under the River Danube. While three additional sections — the first, an eastern extension to Bosnyák tér, the second west to Virágpiac, and a third further east to Újpalota — have been planned, these remain unfunded by the Budapest city government and the European Union.

Before Line 4 was built, only Line 2 served the Buda side of the river. Daily ridership has been estimated at 185,000-195,000 The line operates using fully automated Alstom Metropolis train sets, which are also used on Line 2, although on line 2 the trains are 5 cars in length and have a cab, while on line 4 the trains are 4 cars in length and do not have a cab (but still have the cab doors)

In Hungary, the construction of the line has been widely criticised because its route was perceived as outdated, although the general city-structure and population density remained unchanged. The line has been noted for its high costs and inordinate delays — 17 in total — during construction.