Line 4 is an underground rapid transit line in Milan, Italy, part of the Milan Metro. The line color is blue. The first section opened on 26 November 2022 and as of 2023 it is 7.3 km long with 8 stations. The full line is expected to open by 2024. Once completed, the line will be 15 km (9.3 mi) long with 21 stations.

The Milan Metro is the rapid transit system serving Milan, Italy, operated by Azienda Trasporti Milanesi. The network consists of 5 lines with a total network length of 104.1 kilometres (64.7 mi), and a total of 113 stations, mostly underground. It has a daily ridership of about 1.4 million on weekdays. The Milan Metro is the largest system in Italy for length, number of stations and ridership; and the seventh longest in the European Union.