The S4 is a commuter rail route forming part of the Milan suburban railway service (Italian: Servizio ferroviario suburbano di Milano), which converges on the city of Milan, Italy.

Like all other Milan suburban railway service routes, it is operated by Trenord.

Line S4, a radial route, runs from Camnago-Lentate, in a southerly direction via the Milan–Asso railway, to Milano Cadorna, the railway's urban terminus.

[3] The S4 was activated on 12 December 2004 between Seveso and Milano Cadorna.

[4] On 19 February 2006, the line was extended from Seveso to Camnago-Lentate.