The QF 4-inch gun Mk VI was an Elswick Ordnance Company (EOC) design that was used aboard a few British flotilla leaders and smaller warships during the First World War. A similar EOC gun was designated as the QF Mk X.
The Mk VI was an EOC "Pattern S" gun that that served as the primary armament of the Chilean s built during the early 1910s. Four of these ships were still under construction when the war began in August 1914 and were purchased by the Royal Navy where they were known as the . Only 29 guns were built; it's ammunition was incompatible with other British guns. The submarine was equipped with the gun after she was converted into a patrol boat in 1918. The was also equipped with the weapon.
A single EOC "Pattern Q" gun of similar design was purchased and designated as the Mk X. It is doubtful that it was mounted on a ship.