The British Mark IV Tank was introduced in 1917. It significant improvements over the Mark I Tank, and was the next step in tank development and deployment (the intervening designs being small batches used for training).
The “Male” tanks were armed with three machine guns and two 6-pdrs. Whilst the “Female” tanks had Five .303 Lewis machine guns. Additionally there was an unarmed version used to carry supplies.
The Mark IV was first used in mid 1917 at the Battle of Messines Ridge. It remained in British service until the end of the war, and a small number served briefly with other combatants afterwards.
I have been working on a metal 15mm Mark IV Tank for use with the Home Guard. Though I do really like the Flames of War Great War models, which have a lot more detail.
I also have a Bolt Action 28mm Mark IV Tank on the workbench.
I also have a gallery of Mark IV tanks from a wonderful 28mm demonstration game at GamesDay 2007.