SM U-4 was one of 329 submarines which served in the Imperial German Navy during World War I.
The boat was built at Kaiserliche Werft Danzig and commissioned on 1 July 1909. During the war she served as a training boat and did not see active service. After the war U-4 was partially broken up at Kiel in 1919, with her hull being sold on to Hugo Stinnes the following year.