Good news for all unlocked iDevice users with old basebands and waiting to use Redsn0w / PwangeTool for untethered iOS 4.3.1 jailbreak. The fix is developed by msft_guy and iH8sn0w. Ultrasn0w was previously unable to unlock iPhone 4 and 3GS devices after untethered iOS 4.3.1 jailbreak. But now you can use Ultrasn0w 4.3.1 fixer to unlock your device.
The steps are as follows.
Step 1: At first launch Cydia in your jailbroken iPhone 4 or iPhone 3GS.
Step 2: Navigate to “Manage” and then “Sources”.
* Now “Edit” and then “Add”.
* Type the repo: “http://cydia.pushfix.info”
* Now “Add Source”
Step 4: Now press “Done”.
Step 5: Now select ”Pushfix.info” to Install it.
Step 6: Once its done with installation. Go to Pushfix.info. Scroll down to find “Ultrasn0w fixer for 4.3.1”. Select and to install the package. After that it will restart your device.