And neither should the PZIV frontal armor be invurnable for the 75mm oh and the stug aswel
All game penetrations are based on 500m. At that range, M3 gun with AP ammo can penetrate only 73mm RHA. Since as a rule, weakened armour is not taken into effect, the PzIV hull will remain invulnerable. Don't like that? Tough. I don't want to see this brought up again or you will start to lose man points.
Anyhow, I just thought I'd point out that while the PzIV has thicker frontal armour than the Sherman ingame, it also has fewer hitpoints - which means that comparisons between things killing them are more complicated.