Line S 4 is an S-Bahn of the Rhine-Ruhr S-Bahn, running from Unna to the Dortmund suburb of Dortmund-Lütgendortmund. It is operated by DB Regio on behalf of Verkehrsverbund Rhein-Ruhr. It is operated at 20-minute intervals using class 422 four-car electrical multiple units.
Line S 4 runs over several lines:
S-Bahn services commenced between old Lütgendortmund station and Unna station on 3 June 1984. The section from Germania to old Lütgendortmund station was closed on 31 August 1987 and operations on the new line to Lütgendortmund commenced on 23 May 1993.
Extension of the S 4 line from its current western end in the tunnel at Dortmund-Lütgendortmund was formerly planned to be implemented with a target of opening it in 2015. This would have included an extension of the single-track tunnel to the immediate south of Dortmund-Bövinghausen station. The S 4 would run from there to Herne on the Duisburg-Ruhrort–Dortmund railway (the Emscher Valley Railway of the Cologne-Minden Railway Company) and there take over the Herne–Essen branch of the current S 2 line. The Emschertal-Bahn (RB 43) Regionalbahn service would only operate between Herne and Dorsten. The eastern section of the Emscher Valley Railway between Dortmund-Bövinghausen and Dortmund Hauptbahnhof would be used by a new line of the Dortmund Stadtbahn. For a long time funds have been available for the construction of the line to Bövinghausen, however, the financing for the operation of the line is not guaranteed. The Emschertal-Bahn runs between Dortmund-Bövinghausen and Herne every hour, but the S2 would run between Herne and Essen every twenty minute as an S-Bahn service, so the Verkehrsverbund Rhein-Ruhr would have to fund the increased services through savings in other areas. Upon taking office, the transport minister Oliver Wittke called for all such proposals to be re-evaluated. The line extension has been removed from the integrated transportation plan for North Rhine-Westphalia; a resubmission is possible from 2015.