Versicle homogeneous steel plate with 80mm of thickness was theoretically "immune" to Soviet 76 and US 75 at 500 meters and over--but I am sure a hit at 700m would be unpleasant. I just cannot find any literature about Pz. IV being a problem for Allied tankers. Though one can attribute this to Tiger phobia, there are cases of Panzers positively identified as Mark IVs and 75mm guns did not have any undue trouble against these targets.
As someone else said already, with cherry picking one can make Sherman and T-34 inferior to the Pz. IV, but matching generation with generation, the Mark IV was clearly an inferior tank. The Western Allies failed to up-gun enough of their Shermans for Normandy and so the 75mm M4s had to slug it out with Pz. IV's high velocity gun. Even so, both tanks could kill each other at regular combat ranges.
I used to think the Germans standardized used 80mm armor on several types of their tanks due to both tactical and manufacturing reasons. However, the side hull armor of the Pz. VI E was actually 82mm thick whereas the Pz. IV and Pz. V front plate armor were 85mm thick so the ease of production might not have been what the Germans had in mind.
Btw, I now believe that Mark IV H armor was not face-hardened. The Germans considered face-hardening the Panther tank's glacis armor but apparently 85mm armor was too thick so they opted for rolled homogeneous. If so, there would be illogical for Pz. IV to have FH armor. Somebody on this forum posted a Soviet technical AAR compiled after the Battle of Kursk on my thread about M3 75mm L/40 gun versus Pz. IV, and it states that German medium and heavy tanks had abandoned face-hardening in favor of thicker armor, either as a cost-saving measure or German armor manufacturing method was inadequate to harden steel plates that thick.
It would have been better for the Western Allies had the Germans face-hardened their 82mm and 85mm plates. Than they would be easily defeated at 1,000 yards. RHA was more effective defense when your enemy have decent ballistic cap... Bojan
I'd love to see the "Yugo gun versus Panzers" thread but it seems to be unavailable currently; I can see some of your material by google search, but they're on "best 60s MBT" thread and I cannot find data on US 75mm APC-HE rounds. If the data is in hand, please share!