In the early 1960s these two lines were the two busiest on the network, with loads of 140% of capacity during the evening peak. However, as this change alone was insufficient to overcome this saturation, the length of stations was lengthened from 75 to 90 metres, allowing the lengthening of train from five to six carriages. This work was carried out very quickly and, as early as October 1965, trains of six carriages traversed the line. On 3 October 1966, the first train composed of MP 59 electric multiple units operated on the line. The Line 4 trains were identical to those on Line 1, being composed of four motor and two trailers per train. The line's MP 59 fleet included 556 carriages, comprising 376 powered carriages and 180 trailers. On 17 July 1967, the last steel-wheeled train left the line to strengthen the service of other with a hundred cars built before 1914 scrapped. In February 1971, line 4 was the second network after line 11 to be equipped with semi-automatic operation, with a system known as "Grecque" ("Greek"). This allowed trains once started by the driver to run automatically and stop at the next station.