How to unlock iPhone 4

Note: This guide is for iPhone 4 running iOS 3.1.3 to 4.0. If you are running iOS version later than 4.0. This guide is not recommended for you.
The credit for this unlock solution for iPhone 4 goes to iPhone Dev Team. Without engaging you in more details, I’d guide you through 10 steps to unlock your iPhone 4. It shouldn’t take you more than 5 minutes.
Let’s start:
1. Connect your iPhone 4 to a wi-fi network. No 3G please.
2. Open the Safari browser on your iPhone 4.
3. Go to Jailbreakme.com and jailbreak your iPhone 4. Make sure you have the iPhone firmware 3.1.3 to 4.0. No less than 3.1.3, no 4.x.
4.  Now go to Settings>General>Network and turn off 3G. That’s why I asked to used a wi-fi connections in the first step.
5. Now that you have jailbroken, you have ‘cydia’ installed on your iPhone 4. Press the home button and find ‘cydia’ in your apps in the springboard.
6. In cydia go to search and type ‘ultrasn0w‘. If you are presented with version 1.0.1 or higher skip to step 10 below. If the ultrasn0w version is less than 1.0.1, follow all the steps below.
7. You can’t unlock your iPhone 4 with an ultrasn0w version less than 1.0.1. You’d then need to bring over 1.0.1. For that purpose, press the ‘Manage‘ button at the bottom, select Sources>Edit>add. Then type ‘repo666.ultrasn0w.com’ in the text field without qouts.
8. Reboot your iPhone 4.
9. Go to cydia again, search for ultrasn0w. It must be on version 1.0.1 or higher now.
10. Now press the install button to install ultrasn0w.
11. Your iPhone 4 is now unlocked! Cheers!