SM U-4 or U-IV was a U-3-class submarine or U-boat built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K.

The double-hulled submarine was just under 139 feet (42 m) long and displaced between 240 and 300 tonnes (260 and 330 short tons), depending on whether surfaced or submerged.

The boat was commissioned into the Austro-Hungarian Navy in August 1909, and served as a training boat—sometimes making as many as ten cruises a month—through the beginning of the First World War in 1914.

U-4 was built as part of a plan by the Austro-Hungarian Navy to competitively evaluate foreign submarine designs from Simon Lake, Germaniawerft, and John Philip Holland.

[10] On 27 September 1914, U-4 began operating reconnaissance cruises out of the naval base at Cattaro under the command of Linienschiffsleutnant Hermann Jüstel.

[11] In late November, U-4 seized the 13 GRT Albanian sailing vessel Fiore del Mar as a prize off Montenegro.

[14] Twelve men on Dublin died in the attack,[15] but the cruiser made her way safely, albeit damaged, back to port.

On 3 January 1916, operating again near the Gulf of Drin, Singule and U-4 seized another Albanian sailing vessel, Halil, and sank two smaller boats.

[20] In early February, U-4 sank the 475 GRT French patrol vessel Jean Bart 6 nautical miles (11 km) southwest of Cape Laghi, off Durazzo.

[9][Note 5] Three days later, U-4 sank the British schooner John Pritchard Of Carnar with explosive charges off the island of Antipaxos.

A week later, U-4 successfully torpedoed and sank the Italian schooner Ponte Maria off Brindisi and weathered another unsuccessful enemy submarine attack.

On the night of 14/15 May, the Austro-Hungarian cruisers Helgoland, Saida, and Novara attacked the drifters that deployed the anti-submarine nets that formed part of the Barrage, sinking 14, damaging 5, and taking 72 prisoners.

As a result of the attacks the drifter line of the Barrage was moved farther south and maintained only during the day, a success for the Central Powers.

[29] On 19 June, U-4 scored a triple victory when she sank the French steamers Edouarde Corbière and Cefira and the Greek ship Kerkyra off Taranto.

[23] U-4 was the longest serving Austro-Hungarian submarine,[3] and sank a total of 14,928 GRT and 7,345 tons enemy shipping during the war.