Approximately 250 were built at several shipyards at Leningrad, Murmansk and Astrakhan from 1937 to 1943 and quite equally deployed mainly to the Baltic and Black Sea Navy.
The ship had no armor and a wooden structure instead, but consisted of nine sections isolated from each other, allowing to stay afloat even after suffering heavy damage.
There were cases when the boats arrived the base even without a bow. Being invulnerable due to small displacement (56 tons), small sizes and maneuverability, these ships were intended for operations against enemy submarines in coastal areas.
A tiny "MO" did not capsize in storm and could easily ride the waves making it the most versatile of all coastal war ships.
Three GAM-34BS with a power of 850 hp each yielded a top speed of remarkable 25.5 knots and the underwater exhaust of the engines reduced the noise coming from the boat making it extremely valuable for sudden and covert operations, especially by night.
Armamant consisted of two depth charge launchers, two 45 mm 21-K semi-automatic guns and two 12.7mm DShK hMG.
During the war the MO-4 not only excelled in submarine chasing.
They landed troops and intelligence officers in the rear of the enemy, destroyed the enemy's weapon emplacements; were on patrols and guarded fairways; mined the enemy's coasts, often entering unequal battles with the enemy boats and their aircrafts.
Finally, tireless MOs guarded vehicles in convoys, escorted submarines to the point of immersion and met them after the campaign.