Line 4 is one of the sixteen lines of the Paris Métro rapid transit system and one of its three fully automated lines. Situated mostly within the boundaries of the City of Paris, it connects Porte de Clignancourt in the north and Bagneux-Lucie Aubrac in the south, travelling across the heart of the city. 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 express 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.

The Paris Métro, operated by the Régie autonome des transports parisiens (RATP), is a rapid transit system in the Paris metropolitan area, France. A symbol of the city, it is known for its density within the capital's territorial limits, uniform architecture and unique historical entrances influenced by Art Nouveau. The system is 226.9 kilometres (141.0 mi) long, mostly underground. It has 308 stations of which 64 have transfers between lines. There are 16 lines, numbered 1 to 14, with two lines, Line 3bis and Line 7bis, named because they used to be part of Line 3 and Line 7, respectively. Line 1, Line 4 and Line 14 are automated. Lines are identified on maps by number and colour, with the direction of travel indicated by the terminus.