I would like to suggest the Extra Roof armor for the Turret as a researchable modification for the in Game Panzer IVs, which was a late war development, for increased Roof armor a couple of Panzer IVs and earlier models could be seen with 1-2x 10-13mm plates on the Turrert roof.
This appears to be a workshop modification with the plates taken from older Pz IVs and other sources.
There doesnt appear to be any particular history to it and vehicles in game that could recive this upgrade in form of a modification would be the Pz IV Ausf.G, H, J and Bef.Wg. IV Ausf.J
While the same modification was made for the Panzer V Panther with intention of beeing a stopgap to the Ausf.F, such a connection to (for example the Pz IV Turn Vereinfacht, which was abandond) has not been confirmed/found (except some forum assumtions).
In game this would add more survivability against airtargets and artillery alike.
Be it the Artillery module or enemy HE shells directed at the roof or against the commanders copular.
Depending of the effective armor against the incoming shell, it may not offer mutch but if at least it would add to the extra look of the tank.
Was there a big slat armour that the Germans used in WW2 that would cover the whole tank and make it immune to all damage from Soviet weapons? If so let’s get rid of Russian bias. Yay
It’s probably only for Pz IVs which didn’t already have thickened turret roof armor.
In-game the Pz IV H and J already have increased armor on the turret.