iPhone 4 iOS 4.2.1 Jailbreak is now possible by GreenPois0n RC5 but many people are afraid to update because they might lose the Ultrasn0w unlock. There is a work around to preserve Ultrasn0w unlock ofModem Firmware 01.59.00 and update to iOS 4.2.1 on your iPhone 4. Preserving the unlock will enable you to use iPhone 4 on different carriers without any restriction. You can follow our guide on how to preserve unlock and update to iOS 4.2.1 on iPhone 4.

Warning: This method only works for iPhone 4.

Requirements:
Unlocked iPhone 4 on any older iOS version than iOS 4.2.1.
Tinyumbrella ( Download from here)
iPhone 4 iOS 4.2.1 (Download from here)

iPhone 4 iOS 4.2.1 update without BaseBand Upgrade

Step 1:
Run TinyUmbrella.exe

If Windows shows any Firewall notification, select both check boxes and click Allow Access. (see snapshot below)

Step 2:
Your iPhone 4 should be connected to PC and make sure TinyUmbrella has detected the type of device and its iOS version.

Note: Also save your iOS 4.2.1 or whatever iOS version you are running save its SHSH Blobs by clicking onSave SHSH. This will ensure we can downgrade back incase something goes wrong

Step 3:
Click on Start TSS Server and make sure it does not give any error.

When TSS Server is "running” successfully, TinyUmbrella will force quite iTunes and leaving TinyUmbrella running. Make sure the TSS Server is running by checking TSS server is running under the Log tab.

Warning: This is very important! If you face any error till now DO NOT continue. Your restore will not work if the TSS server cannot be started.

Step 4:
Now run iTunes.

We will have to manually update to iOS 4.2.1. Hold down Shift key (on keyboard) and click Restore

Browse for the iOS 4.2.1 IPSW file you downloaded at the beginning of this tutorial and click Open.

Now Click Restore to begin update your device to iOS 4.2.1.

Step 5:
iTunes must give error 1013. We want this error because it ensures the baseband has not updated.

Click Ok and close iTunes. You can now skip to Step 7.

Important: If you get a 16XX error then try entering DFU mode by following Step 6 and then go back toStep 4. If you did not get error 1013 after restore, it means your baseband was updated. This happens when the TSS server was not running OR you updated without TinyUmbrella OR you are using any OTHER device than iPhone 4.

Step 6: (only if you get an 16XX error after restore)
You need to put iPhone 4 in DFU mode. Hold down both the Power and Home buttons for 10 seconds, then release the Power button and continue holding the Home button for another 5 seconds.

Jailbreak iPhone 4 iOS 4.2.1 using GreenPois0n RC5

Step 7:
We are almost done. Now you can use GreenPois0n RC5 to Jailbreak iOS 4.2.1 and bring your iPhone 4 out of DFU Mode. Follow our step by step iOS 4.2.1 Jailbreak guide here.

**Note: You do not need to put the device into DFU mode after press the Jailbreak button in Greenpois0n as we have already done this 

Unlock iPhone 4 iOS 4.2.1 using Ultrasn0w Unlock

Step 8:
Congratulations, you have successfully updated to iOS 4.2.1 without upgrading your BaseBand version. Now unlock your old baseband using this Ultrasn0w tutorial.