Mark IV was an up-armoured version of the Mark I that went into production in May 1917. Fundamental mechanical improvements had originally been intended, but had to be postponed, the main change consisting in the switch to shorter-barrelled 6-pounder guns. It had all fuel stored in a single external tank (located between the rear track horns) in an attempt to improve crew safety. The sponsons could be pushed in to reduce the width of the tank for rail transportation. 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.