The M4 project or "Blue Line", which saw Webuild as agent of the awarded ATI, involved building a fully automated, driverless, light metro that runs from Linate airport to Milan's San Cristoforo railway station.

The new line, built entirely underground, for a length of approximately 15 km, from Linate to San Cristoforo, is a fast way to travel along the east-west route, paassing under the historic city center.

The contract involved the design (definitive and executive) and construction of two single-track tunnels, one in each direction, with 21 stations, 30 associated structures and a depot/workshop.

The new line is mainly developed in the Municipality of Milan, encroaching, near the eastern terminus, for a small portion into the area of the municipalities of Segrate and Peschiera Borromeo. There are two intersections with the existing metro lines – with the red line near the station of San Babila and with the green line near the station of Sant’Ambrogio – three intersections with the suburban railway lines – at the Forlanini FS station, the Dateo station and the San Cristoforo station – and one intersection with Linate airport.