How to unlock my iPhone 4 for use with other carriers?

I have an iPhone 4 with AT&T and I'm still under contract but its just way to expensive for me so I want to switch to Metro PCS but was told that the phone has to be unlocked. How do I do this and is it free?

You should try out http://unlocksexpress.com/ its only $4 and will be done in less than a day.

I have used allot of unlocking softwares that did not work, but the one that I did find that worked was iphonunlocker.com. It cost $9.99, but is well worth it.

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