Line 4 will be 15km long with 21 stations and will run from Linate Airport to San Cristoforo. Impregilo and Astaldi will be responsible for the civil works which will cost about Euros 870m. Ansaldo STS's share of the contract will be Euros 225m and it will be responsible for installing a communications-based train control system with driverless operation, Tetra radio, WiFi, power supply, platform screen doors, fare collection, and Scada. AnsaldoBreda will supply the trains.

The project will be funded two-thirds by the Italian government and the city of Milan.