The S4 line number was used by the Berlin S-Bahn between December 1997 and June 2002. The line acted as a temporary route for the re-opened parts of the Berlin S-Bahn ring (Ringbahn in German) until the complete ring was fully re-opened in June 2002. The ring is now served by lines S41 (clockwise) and S42 (anti-clockwise).
The S4 commenced service between Jungfernheide and Schönhauser Allee in December 1997, travelling along the re-opened section of the Berlin ring. In May 1998, the line was extended to Bernau. When the ring between Jungfernheide and Westhafen was completed in December 1999, the S4 was extended by an additional two stations. The final alteration to the route came in September 2001 when the line was cut back from Bernau to run for the first time as far as Gesundbrunnen. The line was removed from service in June 2002 when the final ring station, Wedding, was re-opened. The line was replaced by the S41 and S42 lines.
Station listings for the line can be seen from current lines such as the S2 (Bernau-Gesundbrunnen), the S41 and S42 (for the Ring section of the line).