How to Unlock iPhone 4 and iPhone 3GS on iOS 6.1
The evasi0n iOS 6.X jailbreak finally offered a jailbreak solution for iOS 6 devices, including extremely hard-to-jailbreak devices like the iPhone 5, iPad mini, and iPad 4. The team has also just released a new application on Cydia that allows you to unlock the iPhone 4 and iPhone 3GS. The iPhone 5 and iPhone 4S are not supported, as the basebands on those smartphones will not work with the tool. LetsunlockiPhone has put together the requirements and a step-by-step tutorial on how to unlock your iPhone. You'll have to make sure your iPhone 4 and iPhone 3GS is running a baseband that will work with the unlocking tool.
Supported Ultrasn0w Fixer for iOS 6.1 Baseband
Step 1. After jailbreak you need to install Ultrasn0w 6.1 package. To perform this open Cydia => Manage => Source => Edit=> Add this repo:
Step 2. Go to Search, find official Ultrasn0w Fixer 6.1 and install it.
Step 3. Go to Search again and install official Ultrasn0w 1.2.8 Download and Install adding this repo:
Step 4. After installation process you need to restart your iPhone.
Step 5. After device will reboot; wait for the signal.
After you get the signal of your network, your iPhone 4 and iPhone 3GS will be running Ultrasn0w fixer unlock iOS 6.1 device.
You can find more information at Letsunlockiphone.