If you select charge "4" it means you have packs of propellant 1 through 4 in the casing and packs 5 through 7 are outside of the casing. That´s why it shows "5", since it would be a PITA to model all of them.

The number shown on the model is always 1 more than the actual charge you set. Unless you choose charge 7, in which case there is no leftover and thus nothing is modelled.

ah ok thought it would leave the amount of charges outside :D thats how i read it.
so the tag would show 7- charge.

Precisely that, yes :D
The normal procedure is to prepare the round, leave the unused charges outside.
The fire commander checks the chargers before inserting into the gun, tears them off and then the round goes in the gun. So this somewhat simulates it.