How to unlock iPhone 4 from Orange (EE) to Virgin?

My sisters contact expired so she upgraded to the new 5s and gave me her old 4 (not the 4s)

So, what's the best/easiest/cheapest method to get the phone working with my new Virgin sim?

Ask your sister to take it to the carrier and get it unlocked for free. They'll do it since the contract is over for the iPhone 4 and she upgraded to iPhone 5S.