The Sentinel AC IV was a version of the Australian Sentinel AC I cruiser tank, modified to carry a 17pdr high velocity anti tank gun.
The original AC I was armed with a 2-pounder anti-tank gun, already obsolete by the time the tank entered production in the summer of 1942. The AC III was developed to solve this problem, and carried a 25pdr howitzer. This required the production of a new larger cast turret, and an increase in the size of the turret ring from 54in to 64in. Other changes included the use of a common crankshaft, which saved space and allowed the installation of more fuel, and the removal of the bow machine gun and associated crew member.
The prototype AC III underwent trials in February 1943, and was accepted for production, although Sentinel production ended before this happened.
Although the 25pdr armed AC III would have been a good close support tank, it would have lacked anti-tank capabilities. Work thus began on fitting the high velocity 17pdr anti-tank gun in the Sentinel.
In March 1943 the AC III prototype was armed with two 25pdrs in a co-axial mount, in order to test the impact of the high recoil forces generated by the 17pdr. The Sentinel passed these tests, and later in 1943 the 17pdr was installed in a prototype, with the designation AC IV. The AC IV underwent tests late in 1943, but by then the Sentinel programme had already been cancelled, and excellent AC IV didn't enter production.