Redsn0w 0.9.6B4 can jailbreak iOS 4.2.1 on iPhone 4 but for that you need to update your iPhone 4 to iOS 4.2.1 which will upgrade its baseband leaving you in locked state since Ultrasn0w doesn’t support iOS 4.2.1 baseband yet.
If want to unlock your iPhone 4 4.2.1 iOS then either you wait till Ultrasn0w iOS 4.2.1 unlock tool is released or create iOS 4.2.1 custom firmware using PwnageTool to preserve baseband. You might argue PwnageTool for iOS 4.2.1 is not released then how we can create custom firmware ? Well using unofficial PwnageTool Bundles.
The guys over at FSMdotCom have come up with their own PwnageTool bundles for iPhone 4 iOS 4.2.1 which can be used to create custom firmware to jailbreak iPhone iOS 4 4.2.1.
Please keep in mind: The jailbreak will be still tethered one and you won’t be able to use Redsn0w 0.9.6B4 to boot into tethered mode. This method is not tested also we are not sure how it may affect your iPhone 4. You should only proceed at your own risk. We will not be responsible for any damage to your iPhone 4. This guide is only for Mac users.
How to Jailbreak iPhone 4 4.2.1 iOS with PwnageTool Bundles
1. Download all the required stuff and put it on your desktop.
2. Right Click on PwnageTool App & select shown Package content & Browse to Contents>Resources>Firmware Bundles> and copy the iOS 4.2.1 bundle you downloaded
3. Now close the window, load PwnageTool and create your custom firmware for iOS 4.2.1 ( Do make sure you install OpenSSH in the process ).
4. Once you have the custom firmware, start iTunes and click alt+restore ( hold the alt/option key and click on restore )
5. Browse for the iOS 4.2.1 custom firmware you just cooked in step 3, select it, and wait for your iPhone 4 to be restored to this firmware.
6. We have done it. Now you have a jailbroken iPhone 4 running on iOS 4.2.1 – except your iPhone won’t boot in the jailbreaking kernel and.
Here is guide to tethered boot your iPhone 4 iOS 4.2.1 which will be required every time you reboot your iPhone 4.
How to Tethered boot iPhone 4 4.2.1 iOS
1. Download & install MacPorts
2. Install libUSB. Open terminal
sudo port install libusb
3. Duplicate your custom firmware and create a folder on your desktop called ‘ Tethereboot’ ( or whatever you want to call it – doesn’t matter )
4. Download the Thetherboot utility and put it in the Tethereboot folder
5. Take the duplicate custom firmware, change the extension from .ipsw to .zip and unzip
6. Inside there’s two files that you need:
Take them out and put them in the Tetherboot folder alongside the tethereboot utility you just downloaded
7. Open Terminal , put your iPhone in DFU mode and type:
./tetheredboot iBSS.n90ap.RELEASE.dfu kernelcache.release.n90
8. Ok, that’s it. Your iPhone 4 will boot into the tethered boot mode
9. Follow our guide to Manually update & install new Cydia 1.0.3366 on your iPhone 4 iOS 4.2.1.
Thanks to FSMdotCom for this guide and required stuff.
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