M4 is one of the 4 lines of the Bucharest Metro. It is currently the shortest, at only 6.2 km (3.9 mi) long and also the newest. It runs from Gara de Nord to Parc Bazilescu, following the Griviței and Bucureștii Noi avenues.
Bucharest Metro Line M4 Wikipedia
Construction of the line started in 1989, shortly before the Romanian Revolution. The tunnels were built up to where Parc Bazilescu is today. Construction was abandoned afterwards and was resumed later on in the 90's. The first section of the M4 opened on 1 March 2000 from Gara de Nord to 1 Mai. After many years of delays the next section to Parc Bazilescu was finally opened on 1 July 2011. Parc Bazilescu was not in the initial plan and was only built to open the section quicker.
In 2013, construction started on the final 1.6 km (1.0 mi) section of the M4 to Străulești, along with Străulești depot. On 3 September 2015 tunneling work was completed and the two new stations are expected to be open by late 2016. Although the final two stations were finished as of December 19, 2016, Metrorex decide it to postpone their opening until the first half of 2017 due to safety issues.
Despite being the newest line on the network, the M4 is the only line to run Astra IVA trains. They're going to be replaced after Metrorex replaces the signalling system from Dimetronic with ones from Bombardier.