The 1.5km extension from Porte de Orléans is the first addition to Line 4 in more than 100 years and is expected to boost ridership on the line by around 37,900 passengers per day. The journey time between Mairie de Montrouge and the northern terminus at Porte de Clignancourt is around 30 minutes.

By the end of 2019, Line 4 will be extended a further 1.7 km to Bagneux with new stations at Verdun Sud and Bagneux.

Line 4 is the second-busiest Metro Line in Paris, carrying around 674,000 passengers per day.