Can you unlock a Iphone 4 if its from a different country?

I got a IPhone 4 but its from the United Kingdom so if i unlock it can i still use it on a United States carrier?

As long as its not blacklisted, you can unlock it
since its from the UK, its a GSM phone, and once unlocked, you can install a SIM from any GSM carrier (AT&T and T-Mobile are the largest)

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