MO-4 is a class of small ships produced before and during World War II for the Soviet Navy. Their primary function originally was anti-submarine warfare.
MO-4 was the most produced type of the class, with approximately 250 built. It was a slightly modified MO-2 variant, with a more vertical stern and slightly lower.
It was designed at the NKVD “No. 5 construction bureau” in Leningrad. The ship had no armor and had a wooden structure, but consisted of nine sections isolated from each other, allowed to stay afloat even after suffering heavy damage.
MO-4 ships were built at several shipyards including Leningrad, Murmansk and Astrakhan from 1937 to 1943.