How to unlock sprint iphone 4 to verizon?

I got an iphone 4 from my mother, because she got a sign up deal or free upgrade {i forgot which} and got herself another iphone for herself. My current phone is several years old and failing. My mother said if i could figure out how to get it to work for verizon than i could keep it?
so does anyone know how to unlock a to sprint iphone 4 and make it work for verizon?

You should get it unlocked for free from Sprint as the contract is over

You can't. Verizon will not activate another carrier's phones on its network.