QF 4 inch naval gun Mk IV, XII, XXII

The QF 4-inch gun Mk IV was introduced in 1911 as a faster-loading light gun successor to the BL 4 inch Mk VIII gun, and was the main gun on most Royal Navy and British Empire destroyers in World War I.

Mk IV gun
Mk IV armed many British destroyers and some cruisers in World War I. It was used to arm merchant ships in World War II.

The guns armed the following warships :
 * Forward class scout cruisers as re-gunned in 1911
 * Sentinel class scout cruisers as re-gunned 1911-1912
 * Pathfinder class scout cruisers as re-gunned 1911-1912
 * Adventure class scout cruisers as re-gunned 1911-1912
 * Acasta (K) class destroyers of 1911
 * Laforey (L) class destroyers of 1913
 * Yarrow M class destroyers laid down 1912 - 1915
 * Admiralty M class destroyer of 1913
 * Thornycroft M class destroyers laid down 1913 - 1915
 * Hawthorn M class destroyer of 1914
 * Talisman class destroyers of 1914
 * Medea class destroyers of 1914
 * Faulknor class leaders of 1914
 * Marksman class destroyers of 1914
 * Parker class leaders of 1915
 * Yarrow Later M class destroyers of 1915
 * R class destroyers of 1916
 * S class destroyers of 1917
 * Fundy class minesweepers of 1938 (guns from decommissioned Canadian S class destroyers)

Mk XII and XXII submarine gun
The Mk XII variant was developed for arming submarines from 1918, Mk XXII was developed to arm submarines during World War II. These submarine guns fired a heavier 35-pound projectile from late 1944. Mk XII and XXII equipped :
 * L class
 * Odin ("O") class
 * Parthian ("P") class
 * River (or Thames) class
 * Grampus (or Porpoise) class
 * Triton ("T") class
 * S class
 * Some of the Amphion ("A" or Acheron) class

Surviving guns

 * The Mk IV gun from HMS Lance which fired the first British shot of World War I on 5 August 1914 is on display at the Imperial War Museum, London.

Weapons of comparable role, performance and era

 * 10.5 cm SK C/32 naval gun : Slightly more powerful German equivalent WWII submarine gun