Update iPhone 4 to iOS 4.2.1 While Preserving Baseband [How to Guide]

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.
STEP 1: Right click on Notepad from Start - Programs - Accessories, then choose Run as administrator from the contextual popup menu.

STEP 2: Once notepad opens, select Open from the File menu then navigate to C:\Windows\System32\drivers\etc\ and open the hosts file as shown:

STEP 3: Now add 74.208.10.249 gs.apple.com to end of the file as shown in the screenshot below:

STEP 4: Now, you'll 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 .

STEP 5: iTunes should pop-up with some error as error 1013, 1015 or some other error. This indicates that you have done it successfully, hit OK to continue.

STEP 6: Now you iPhone will stuck in Recovery Mode showing the well-known "Connect to iTunes" message. You have to run TinyUmbrella (Windows - Mac) or RecBoot 2.1 for Mac OS X to get out of recovery mode.

You'll now be able to jailbreak your iPhone 4 using Redsn0w 0.9.6 for tethered and Redsn0w 0.9.7 for untethered one then unlock it using Ultrasn0w carrier unlock.