German submarine U-4 was a Type IIA U-boat of Nazi Germany's Kriegsmarine before and during World War II. She was one of the longest lasting German submarines of the period, primarily since half of her time was spent on training duties in the Baltic Sea.

Commissioned on 17 August 1935, she was one of the first batch of Type IIA boats constructed following the Anglo-German Naval Agreement that repealed the terms of the Treaty of Versailles which ended the First World War and stated that Germany was not permitted to possess submarines. Built at the Deutsche Werke in Kiel as yard number 239, she was a highly sought after command before the outbreak of the Second World War in 1939. Once the war had begun however, she rapidly became obsolete. Before she was superseded, she carried out four combat patrols, mainly in support of the Norwegian campaign.

German Type II submarines were based on the Finnish submarine Vesikko. U-4 had a displacement of 254 tonnes (250 long tons) when at the surface and 303 tonnes (298 long tons) while submerged. Officially, the standard tonnage was 250 long tons (250 t), however. The U-boat had a total length of 40.90 m (134 ft 2 in), a pressure hull length of 27.80 m (91 ft 2 in), a beam of 4.08 m (13 ft 5 in), a height of 8.60 m (28 ft 3 in), and a draught of 3.83 m (12 ft 7 in). The submarine was powered by two MWM RS 127 S four-stroke, six-cylinder diesel engines of 700 metric horsepower (510 kW; 690 shp) for cruising, two Siemens-Schuckert PG VV 322/36 double-acting electric motors producing a total of 360 metric horsepower (260 kW; 360 shp) for use while submerged. She had two shafts and two 0.85 m (3 ft) propellers. The boat was capable of operating at depths of up to 80–150 metres (260–490 ft).

The submarine had a maximum surface speed of 13 knots (24 km/h; 15 mph) and a maximum submerged speed of 6.9 knots (12.8 km/h; 7.9 mph). When submerged, the boat could operate for 35 nautical miles (65 km; 40 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 1,600 nautical miles (3,000 km; 1,800 mi) at 8 knots (15 km/h; 9.2 mph). U-4 was fitted with three 53.3 cm (21 in) torpedo tubes at the bow, five torpedoes or up to twelve Type A torpedo mines, and a 2 cm (0.79 in) anti-aircraft gun. The boat had a complement of 25.

U-4 departed Wilhelmshaven for her first patrol on 4 September 1939, in the aftermath of the declaration of war. Given her small size, she only covered the area to the south of Norway and into the North Sea west of Denmark and along the Dutch coast, and returned unsuccessful on 14 September. A second patrol later in the month yielded greater dividends, when she spotted three neutral vessels heading to Britain through the North Sea and sank them on consecutive days; the Martti Ragnar on the 22nd, the Walma on the 23rd and the Gertrud Bratt on the 24th.

Her later two patrols were both in support of the German invasion of Norway. It was during this operation that she was able to sink the British submarine HMS Thistle. The engagement was a lengthy one, as U-4 was a small coastal craft with poor endurance. When Thistle attacked and missed U-4 on 9 April, it gave the German submarine a chance to evade and hunt her attacker, finally catching and sinking the British vessel as she recharged her batteries on the surface a day later.

Once Norway was subdued, it became increasingly obvious that U-4 and her sisters were not capable of either outfighting or outrunning enemy craft, neither did they possess the range and endurance necessary to have a major impact on Allied shipping. In response, on 1 July 1940, these boats were relegated to the 21st U-boat Flotilla, and served until 1944 as training craft in the Baltic Sea. Although some of her sisters saw action later against the Soviets, U-4 did not, eventually being retired from all service at Gotenhafen (now known as Gdynia in Poland), on 1 August 1944, and then scrapped for parts sometime in 1945.

Date	Name	Nationality	Tonnage	Fate
22 September 1939	Martti Ragnar	Finland	2,262	Sunk
23 September 1939	Walma	Finland	1,361	Sunk
24 September 1939	Gertrud Bratt	Sweden	1,510	Sunk
10 April 1940	HMS Thistle	Royal Navy	1,090	Sunk

• Helgason, Guðmundur. "The Type IIA boat U-4". German U-boats of WWII - uboat.net. Retrieved 6 December 2014.
• Hofmann, Markus. "U 4". Deutsche U-Boote 1935-1945 - u-boot-archiv.de (in German). Retrieved 6 December 2014.

54°32′N 18°33′E / 54.533°N 18.550°E / 54.533; 18.550