If you owned the amazing AT&T Galaxy S4, if you’re still on contract, you may then not enjoy network freedom on your device. If you really want to give freedom to your device, then you may consider Sim-Unlock AT&T Galaxy S4. You probably would have seen, many service providers on the web, to Sim Unlock your device, but then it will cost you, additionally, in reality there are only a few, who may unlock the AT&T Galaxy S4.
I should and must give credit to the XDA Developers Community, who are always busy in finding solutions for your problems. Sush143555, an xda, the member who has just discovered a new way to Sim Unlock AT&T GS4 for Free, yes exactly you can carrier unlock it for free. I know you may’ve tried others method to unlock your device, but almost they are all blocked by Samsung. Check out this one, and it may turn your day into a good day :).
Before we move on, I want to make it clear, this guide is only for educational purpose, we here at ohguideme, do not take any responsibility for any consequences that may occur because of this.
How to Sim-Unlock AT&T Galaxy S4
1. First of all the call dialer on your Galaxy S4, and dial “*#0011*” (probably without quotes)
2. Now you need to enter Service Mode on your GS4, a little tricky but try to understand it;
- Press the Menu button followed by the Back button
- Now again hit the Menu Button and select ‘KEY INPUT‘
- At last, enter 1 and press ok to enter service mode.
3. Just one more time :P, hit the Menu button and then tap Back again.
4. Tap  UMTS
5. Then, DEBUG SCREEN, followed by  PHONE CONTROL.
6. Further, choose options,  NETWORK LOCK.
7. On the next menu,  PERSO SHA256 OFF.
8. You’re now supposed to go back to Main Service Mode Menu, hit the Menu button followed by the Back button (tap).
9. Now from the main menu, choose COMMON, and then  NV REBUILD
10. At the end, your GS4 will tell you, ‘a Golden Backup exists‘ just tap the option no. Restore Back-up.
At last you’ll notice some strange and peculiar conditions; like at first your device will freeze then you’ll notice just black screen with Blue LED on and then finally it will automatically gets rebooted. During all these steps, you need to have patience, while your device processes everything, and it can time, depending on your device.
I really wish you the best of luck, in Sim Unlocking your AT&T Galaxy S4, and hoping that everything goes smooth for you, without any errors. Let me know, how it went for you and how much you were successful in your attempt.[source:xda]