Downgrade iOS 4.2.1 to iOS 4.1 / 4.0.2 / 4.0.1 on iPhone 4, 3GS, 3G, iPod Touch 4G, 3G

It was possible that you have the curiosity to experiment iOS 4.2.1 GM new features which was released yesterday to iPhone 4, 3GS, 3G, iPod touch 4G, 3G, 2G and iPad, if you still didn't, you can download and jailbreak it now from here. You will note that Cydia still have some issues, the matter which make jailbreak pointless for many.

Moreover the new iOS 4.2.1 GM build has a new baseband version which puts a lot of doubts around the upcoming unlock. Therefore I prepared the downgrade guide from iOS 4.2.1 GM to iOS 4.1, 4.0.2, 4.0.1 or lower.

STEP 1: Download iOS 4.1 or older firmwares for iPhone or iPod Touch.

STEP 2: Put your iPhone in DFU Mode by following this steps,

• Connect your iPhone to your computer.
• Turn iPhone off.
• Start iTunes
• Hold Power and Home buttons together for 10 seconds or so.
• Release Power button but keep holding the Home button until your computer recognizes a new USB device.
• iTunes will now recognize your iPad.

STEP 3: If you are on iPhone 3G or iPod touch 2G (non-MC model) bypass this step. This step is about adding Saurik’s Server address to your system hosts file. This trick make iTunes thinks it is talking to Apple, while it is talking to Cydia while the restoring process.

• Locate "C:\Windows\System32\drivers\etc\" for Windows and "/etc/" directory for Mac, then locate “hosts” file andOpen it in Notepad (for Windows) or TextEdit (for Mac OS X) and add this at the bottom of the file.

74.208.105.171 gs.apple.com

At last it will be like that, then save it

STEP 4: Now, You will have to restore your iDevice to iOS 4.1, 4.0.2 or 4.0.1  by clicking on the iPhone icon on the left sidebar of iTunes then hold left “alt” button on Mac (“Shift” button on Windows) on the keyboard and then click on “Restore” then release the alt button, Now Select the iOS 4.1, 4.0.2 or 4.0.1 ipsw file you have already downloaded in Step 1 and let iTunes do some automated Steps.

STEP 5: You will need to exit recovery mode, so download TinyUmbrella (Windows / Mac). Run it and the click on “Exit Recovery” button. Your device will now restart normally.

Now you got back to iOS 4.1, 4.0.2 or 4.0.1 and you can jailbreak your iPhone using our guide posted here.