How Do I Unlock My iPhone 4?

[iMore Question]

I bought an iphone 4 locked to AT&T. I called AT&T to have them unlock it but they needed the previous owner's number to do that, which I don't have.

Here are details of my iPhone 4:
Version: 6.1.3
Firmware: 4.12.05

1) How do I unlock it to work on MetroPCS?
2) How do I make sure that the phone I got is an iPhone 4?

 

[iOS Gravity]

Go to the Metro PCS store and ask them to flash the phone to their network.