Make it a weapon that sits in the Shield slot if you already have a weapon.
It still does its 4 damage while there.
So basically you've little bit tweaked the idea Twinbuster originally made:
5) Dual Wield: You could have 2x of this weapon simultaneously, stacked or placed to empty shield slot.
I don't like to add another alternative to the poll when it's so similar with already existing one, but I'll change that Dual Wield option so it also includes your version:
5) Dual Wield: You could have this weapon simultaneously with another: 2x stacked on weapon slot, or second placed to shield slot, or this weapon would go into empty shield slot if weapon slot was already taken with any weapon.
Is that written the way you meant it, or do you want me to change it?