Most of iPhone 4 users didn't update to iOS 4.2.1 as it updates baseband and make it not unlockable. So in case you got an iPhone 4 on older 01.59.00 baseband, we have been tipped off about a solution for that. The below steps will guide you to update iPhone 4 to iOS 4.2.1 while preserving the same older baseband 01.59.00 and unlock it.





Update iPhone 4 To iOS 4.2.1 While Preserving Baseband [How To] :

1) Click on Start > Programs > Accessories > Notepad , right click on it and then choose Run as administrator from the contextual popup menu.

                                                  

2) Once Notepad opens, select Open from the File menu then navigate to C:\Windows\System32\drivers\etc\ and open the hosts file.

                   

3) Add 74.208.10.249 gs.apple.com to end of the file.

                  

4) Now, you will have to restore your iPhone 4 to iOS 4.2.1 by clicking on the iPhone icon on the left sidebar of iTunes then hold left Alt button (Shift button on Windows) on the keyboard and click on Restore then release the alt button, Now Select the original iOS 4.2.1 firmware .

5) iTunes should popup with some error as error 1013, 1015 or some other error. This indicates that you have done it successfully, hit OK to continue.

6) Now your iPhone 4 will stuck in Recovery Mode showing the "Connect to iTunes" message. You have to run TinyUmbrella (Windows/Mac) or RecBoot 2.1 for Mac to get out of recovery mode.

You will now be able to jailbreak your iPhone 4 using Redsn0w 0.9.6 for tethered and Redsn0w 0.9.7 for untethered jailbreak then unlock it using Ultrasn0w unlock tool.

If you have any problems or queries, please feel free to comment below.