The modernization of Line 4 of the Paris Metro has been completed with the highest grade of automation and the automatic driverless metros have now begun revenue service. The digitalization and optimization of Line 4 will allow RATP to safely increase frequency by reducing intervals between trains from 105 to 85 seconds. Chosen in 2015 by RATP to automate Line 4, Siemens Mobility supplied their digitalized communication and CBTC signaling systems to modernize the network. After a trial run with no passengers that began in June, the automation of Line 4 is now a reality. More automatic driverless shuttles will be introduced into the line's traffic over time to achieve full automation by the end of 2023.For this project, Siemens Mobility equipped the tracks, the technical premises and the 52 automatic shuttles that will run on Line 4 with its digitalized systems and CBTC signaling. Siemens Mobility also supplied the equipment and software for the Operation Central Command, which manages the signaling system and the traction power supply, as well as interfacing with the on-board and trackside systems.Siemens Mobility’s digitalized CBTC signaling will allow RATP to safely increase capacity and accommodate more passengers on the system. The ability to continuously receive updates on system status will also improve operational efficiency, resulting in fewer delays and more up-to-date travel information. Additionally, the automated system will reduce traction energy consumption by up to 15%.Line 4 is the second busiest metro line in Paris. It carries 700,000 passengers per day, through 29 stations that are spread over 14 km. The busy line, and the need to minimize the disruption of passenger service, was one of the many challenges brought on by this project. The automation of Line 4 needed to incorporate the work being done to extend the line to its new southern terminal Bagneux-Lucie Aubrac, while also needing to successfully interface with three different types of rolling stock (MP89, MP05 and MP14). All features were successfully carried out despite the challenges brought on by the pandemic.