The Line 4 is a new underground rapid transit line under construction in Milan, Italy, part of the Milan Metro system. The line was planned to be completed in 2015, as part of the works for the upcoming Milan Expo. However, delays in planning, financing and construction works forced to postpone the opening, and the line is now expected to open in 2022.The line will be 15 km (9.3 mi) long with 21 stations. The expected annual ridership is 87 million.