The U4 is a line of the Berlin U-Bahn in Germany that is the shortest in Berlin's U-Bahn system, with a length of .
Operations
The turn-of-the-century predictions turned out to be accurate since ridership on U4 is lower than on other lines in the system, partly because of the short length of the line. All other Berlin U-Bahn lines run at night on weekends, but that is not the case for U4 (the only other line with that distinction was the temporary U55 prior to its connection to U5 in 2020). Nonetheless, a five-minute headway is maintained at peak times, which decreases to ten- or fifteen-minute headways at less busy times of the week.
Extension plans
U4 is the only subway line in Berlin never to have been extended despite plans at the time of its construction that would eventually extend the line (see above). A southward extension would have significant technical hurdles to overcome since the tunnel of Bundesautobahn 100 is in the way, and there seems to be no appetite for removing the section of A100. There were various plans for a modest northward extension towards Magdeburger Platz in the 2000s, but it has ultimately determined that the shift of passengers to the newly extended line would be unlikely to justify the expense and so those plans were shelved.