I think shields should grant anything up to a +4 armour bonus, but it should follow the default rules for stacking: ie, it doesn't stack with any other armour bonus.
Notice that you don't see massive shields in the age of plate armour, because the armour itself is better at providing protection than any shield could be.
Notice that you don't see massive shields in the age of plate armour, because the armour itself is better at providing protection than any shield could be.