The Mark IV was a more heavily armoured version of the Mark I, and went into production in May 1917. Fundamental mechanical improvements had originally been intended, but had to be postponed. The main change was the introduction of shorter-barrelled 6-pounder guns. It had all its fuel stored in a single external tank (located between the rear track horns) in an attempt to improve crew safety. The sponsons could be swung in on hinges to reduce the width of the tank for rail transportation (previous models required partial disassembly to fit within the loading gauge). Rails on the roof carried an unditching beam. A total of 1,220 were built: 420 males, 595 females and 205 tank tenders, which were supply tanks.
The Mark IVs were used successfully at the Messines Ridge in June 1917, where they outpaced the infantry on dry ground, but in the Third Ypres of July and August they found the swampy ground difficult and were of little use. About 432 Mark IV tanks were used during the Battle of Cambrai in November 1917.
The first tank-to-tank battle was between Mk IV tanks and German A7Vs in the Second Battle of Villers-Bretonneux in April 1918.