The Mark IV tank was the first real improved version of the original tank (Mark 1)
The improvements included more armour and a single external fuel tank. But basically it was still the same tank as first fielded by the British and thus still experienced nearly all the same faults.
Crew of 8 (Male tanks were armed with 2 x 4 Pounder guns + 4 machine guns. Females were armed with 6 machine guns)
It was superseded in service by the vastly improved Mark V