Line 4 (French pronunciation: [liɲᵊ katʁᵊ]) is one of the sixteen currently opened lines of the Paris Métro, as well as one of its three fully automated lines. It connects Porte de Clignancourt in the north of Paris, to Bagneux-Lucie Aubrac in the eponymous southern suburban city, on a north-south axis across the French Capital. Held within the boundaries of the City of Paris for over a century – until its southern terminus was changed from Porte d'Orléans to Mairie de Montrouge in 2013 – the line was sometimes referred to as the Clignancourt – Orléans Line. At 13.9 km (8.6 mi) in length, it connects with all Paris Métro lines apart from the very short 3bis and 7bis branch lines, as well as with all 5 RER train lines. It also serves three of the Paris Railway stations, Gare du Nord, Gare de l'Est, and Gare Montparnasse. It is the second-busiest Métro line after Line 1, carrying over 154 million passengers in 2004.
Line 4 was the first line to connect to the south side of the River Seine, through an underwater tunnel built between 1905 and 1907. Line 4 long ran the longest-serving MP 59 rubber trains, which used rheostats to dissipate the braking power through resistance, making line 4 the hottest line in the system. Those trains were withdrawn from service on Line 4 during the course of 2011 and 2012 after 45 years (with some being in service for 50 years), yet remained another 12 years on in service on Line 11. On Line 4, they were replaced by the MP 89 CC stock transferred from Line 1, right after its automation.
