The S4 is a commuter rail route forming part of the Milan suburban railway service (), which converges on the city of Milan, Italy.
The route runs over the infrastructure of the MilanâÂÂAsso railway. 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 MilanâÂÂAsso railway, to Milano Cadorna, the railway's urban terminus. The travel takes 42 minutes.
The S4 was activated on 12 December 2004 between Seveso and Milano Cadorna.
On 19 February 2006, the line was extended from Seveso to Camnago-Lentate.
The stations on the S4 are as follows (stations with blue background are in the municipality of Milan):
, S4 trains ran every half-hour between 06:00 and 21:00 daily. After 21:00, the connection between Seveso and Milano Cadorna was provided by S2 trains between Seveso and Milano Porta Vittoria, connecting at Milano Bovisa-Politecnico with S3 services to or from Milano Cadorna.