Activate Used iPhone 4
So long as it's an unlocked iPhone, you can use any SIM card to activate the unit. It doesn't actually have to have a plan attached to it. I'm not aware of any carrier on the world that only supplies contracts and foregoes monthly plans, so there should be no issue with your carrier of choice. Contacting Verizon is a wise idea with respect to their policies, as without further details your question is pretty broad.