What Iphone 4s can be use internationally?

Anyone know what iphone 4s can use internationally?

I need to buy an iphone 4s for someone to bring it to Vietnam as a gift to a family member.
But I need to know if I they can use it outside of US.

Do i just buy any Iphone 4s from AT&T and unlock it? Will that make it "world wide"?

I did a lot of googling, but still unsure and no where I can find a positive answer. I just don't want to go on craigslist and buy an iphone 4s they posted as "can be use internationally" and one of my family in Vietnam can't use the phone when they receive it.

Very much appreciated, thank you.

Yea I think you can use any of them imma ask my sister Joanna

You can use any iPhone to use it internationally. Yes, just buy At&t iPhone & Unlock it. Yes, that will make it worldwide. I suggest you to buy only At&t iPhone because other networks unlocking prices are high.

IPhone 4s can be used Internationally in any GSM network only if it is Unlocked.
You can Unlock your iPhone 4s from anywhere. Once your iphone 5 is unlocked you can use it in any country.
You can buy any Iphone 4s from AT&T and unlock it using Remote Unlock Method provided by http://www.unlock-zone.com/apple-iphone-4s/rs16wp9/. They will remotely unlock your iPhone 4 after getting information of the Country and Current Network in use.
It is safe and easy. Once Unlocked, The iPhone 4s can be used in any GSM network available in Vietnam.

Yes if you buy an ATT locked, you can get it unlocked cheap at sites like iPhoneUnlockedCheap.com and it will be unlocked worldwide