Can I use my Verizon Phone with no contract?

Okay I have a iphone 5 and a 4. The contract renewed. Can I still use the 4 with stuff like Skype? I want to give the 4 away but I am afraid that if the person uses it, somehow I will be billed by verzion. Is there anything I can do to make sure that never happens and still give away the phone?

I would be willing to hack the old phone is necessary.
 
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