How to Factory unlock iphone4 16Gb at&t?

My cousin gave me his iphone4 which is locked to at&t USA. I know redsn0w use however with this iphone 7.0.4 iOS - it is returning me an error "Blobs are corrupted". With no help on it, I decided to see options for factory unlock however the price are real high. I heard people getting free factory unlock code, where I can find these, if any exists? I also downloaded a software iPhone 4 unlocker v1.0 however to get serial key for this, I have to complete a survey [which I'm doing but still not getting serial key file from FileIce website]

Any help, especially when I would not have to spend anything?

You can't factory unlock an iphone apple does that
what you can do is jailbreak the phone use evasion software
here is a link
http://evasi0n.com/

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