When everything is changed, it isn't a Mk IV anymore. Hitler proposed a new front with sloped armor for the Mk IV after the T-34 appeared, but it was considered to be too complicated. The Panther was cheaper to produce and had everything what was mentioned in post No.7. A more powerful engine, sloped armor, a bigger gun, wide tracks. In 1943/44 the Mk IV had reached the end of his career as a battletank, it was a successful design of the late 30ies and still capable to deal with the Sherman or the T-34, but it wasn't designed to be mass-produced.