Line 4 () is one of the sixteen lines of the Paris Métro rapid transit system. Situated mostly within the boundaries of the City of Paris, it connects Porte de Clignancourt in the north and Mairie de Montrouge in the south, travelling across the heart of the city. Prior to 2013, when the southern terminus was changed from Porte d'Orléans to Mairie de Montrouge, the line was sometimes referred to as the Clignancourt – Orléans Line. At in length, it connects to all of the lines of the Métro apart from the 3bis and 7bis branch lines, as well as all of the RER express lines. Further, it is the second-busiest Métro line after Line 1, carrying over 154 million passengers in 2004.