Seiner Majestät UB-4 was a German Type UB I submarine (U-boat) in the German Imperial Navy (German: Kaiserliche Marine) during World War I. She was sunk by a British Q-ship disguised as a fishing smack in August 1915. UB-4 conducted the first sortie of the Flanders Flotilla in April, during which she sank the Belgian Relief ship , the first ship credited to the flotilla. She sank three more ships from mid-April to mid-August. On 15 August, UB-4 surfaced near the British Q-ship Inverlyon and was sunk by gunfire from the sailing vessel. None of UB-4's 14 crewmen survived the attack.