[2] Opened in 1910, the U4 serves five stations, all of which are step-free: it is also the only subway line in Berlin to have never been extended and the only one to have no night service on weekends.

In 1903, Schöneberg, an independent city that as south-west of the municipal limits of Berlin, planned to develop an underground railway line to improve public transportation.

The route was intended as an underground railway from Nollendorfplatz, where an elevated train station of the Berlin Hoch- und Untergrundbahn already existed, to the Hauptstraße (main street) in the south of Schöneberg.

Siemens & Halske AEG were tasked with all aspects of construction, including the equipment of the track and the delivery of the vehicles.

However, the festivities were rather subdued since the "father" of Schöneberg's subway, Mayor Rudolph Wilde, had died a month earlier.

Schöneberg thus became the second city in the German Empire to build a subway, ahead of Hamburg U-Bahn, which opened in 1912, and the first to follow the now-common model of financing a project municipally to be realised by private contractors.

The previous method was having all aspects handled by the private sector, with the government acting only as a regulatory and a concession-granting entity.

Since the Schöneberg subway was initially completely separate from the rest of the Berlin U-Bahn network, extra facilities had to be built to accommodate it.

In the course of construction of Bundesautobahn 100, the long-unused tunnel to the defunct depot was interrupted, which significantly hinders any potential extension southward (see below).

The SelTrac system that was used on line U4 was manufactured by Standard Elektrik Lorenz (later part of Alcatel-Lucent), and allowed very tight headways of 50 to 90 seconds.

It was the first trial service of an automatic U-Bahn in Germany, but to alleviate riders' concerns, a driver was still present.

On 27 November 2010, the occasion of the 100th anniversary of the Schöneberg subway, a commemorative service with historical rolling stock (type A1) was run.