A second modification of the route was also made: it was decided to make a temporary deviation via the major station of Gare Montparnasse to avoid a further delay in opening the line, which was eagerly awaited.

[5] In 1905 construction was started by the company of Léon Chagnaud—a former mason from Creuse (a department with a tradition of supplying building workers in France)—and line 4 became the first to cross the Seine underground.

The installation of two refrigeration plants allowed the movement of brine cooled to −25 °C in dozens of tubes to stabilize the ground.

However, the line was closed to traffic a few days later in January 1910, when the level of the Seine broke its banks during the worst flood of the century.

On 26 January Châtelet station and the crossing under the river was completely flooded and slowly rose in the tunnel.

The fall in the level of flooding during February allowed a gradual resumption of operations, but full service was not restored to 6 April after repair of extensive damage caused to the infrastructure.

[7] During the politically turbulent 1930s, Line 4 had its own drama: on 27 July 1934, a package left in a carriage was carried into the office of its chief supervisor, located on the platform of Montparnasse.

[8] During World War II the most violent bombing Paris suffered was on the night of 20 and 21 April 1944 when the rail freight yard of la Chapelle and the main truck workshop at Rue Championnet were hit.

On 17 July 1967, the last steel-wheeled train left the line to strengthen the service of others with a hundred cars built before 1914 scrapped.

The changeover took place on three consecutive nights from 10 PM on Friday, 30 September 1977 to the beginning of services on Monday, 3 October.

In addition to the Montrouge extension, in 2011 there has been a much needed refreshing in rolling stock for Line 4, as the MP 59 trainsets were reaching the end of their useful lives.

Testing of the MP 89 during overnight hours took place in 2010, with the first train (#01) to be transferred to Line 4 in April, 2011 and enter service on 23 May 2011.

[citation needed] Following the opening of Mairie de Montrouge, plans were already in the works to further extend Line 4 another 1.8km southward.

[17] The beginning of the conversion was initially planned shortly after the completion of the Line 1, but was put on hold for a time due to high costs (From fr:Ligne 4 du métro de Paris).

The RATP confirmed on 2 April 2013 that Line 4 would be fully automated, but still stopped short of giving a timeline of the conversion.

[19] Delays partly brought upon by the COVID-19 pandemic however pushed the introduction of mixed rolling stock back to summer 2022.

(From fr:Ligne 4 du métro de Paris) Until March 2013; light maintenance of the MP 89CC rolling stock of Line 4 was handled within the station of Porte d'Orléans (CDT).

Platform 2 of the station (in the direction of Porte de Clignancourt contains inspection pits, which allow routine light maintenance work and troubleshooting of the first level.

This protocol changed when a new garage located south of the new station Mairie de Montrouge opened.

Second level maintenance is handled at the Saint-Ouen shops, located just to the north of the Porte de Clignancourt terminus (and its northern loops).

The shops have been renovated and expanded over the years, including a recent extension to house MP 89CC trainsets that are undergoing routine maintenance (AMT).

All heavy maintenance work for the MP 89CC trainsets are handled in the workshops of Fontenay, located just east of the Chateau de Vincennes terminus of Line 1.

(From fr:Ligne 4 du métro de Paris) (From fr:Ligne 4 du métro de Paris) Metro line 4 passes near several places of interest and therefore often crowded : According to Facebook group New Urbanist Memes for Transit-Oriented Teens, M4 is considered "chaotic good" on the Dungeons & Dragons (D&D) alignment system.