S4, S 4, Š-4, S.4 or S-4 may refer to:
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.
The route runs over the infrastructure of the Milan–Asso railway. Like all but one of the 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.
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 a coloured background are within the municipality of Milan):
As of 2012, 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.
The Sihltal railway line (German: Sihltalbahn) is a railway line (and a former railway company) in the Swiss canton of Zürich, which connects the city of Zürich with the communities of the Sihl Valley. Passenger service on the line now forms part of the Zürich S-Bahn, branded as that network's service S4, and is part of the Zürcher Verkehrsverbund (ZVV) zone-based fare network.
The line was opened in 1892 and electrified in 1924. Today it is owned by the Sihltal Zürich Uetliberg Bahn SZU AG, a company that also owns the Uetliberg line, and organizes the Zimmerberg Bus, and operates the Luftseilbahn Adliswil-Felsenegg (LAF).
The Sihltal line was built by the Sihltalbahn company (SITB), which opened a line from Bahnhof Selnau to Sihlwald in 1892. Bahnhof Selnau was already in existence as the terminus of the Uetliberg line, and the two lines ran in parallel as far as Giesshübel station. Later in the same year, a freight branch was constructed linking Giesshübel station with Wiedikon station on the Lake Zürich left bank line of the Swiss Northeastern Railway (NOB).