As you know GeoHot released Limera1n jailbreak for iPhone 3GS, iPhone 4 and iPod Touch 3G and 4G and Greenpois0n is on hold for a while. Limera1n jailbreak requires you to install iOS 4.1 first and then apply the limera1n jailbreak as instructed here. Direct update to stock firmware is equal to updated baseband. Last month we published a Guide to Update iPhone 4 to iOS 4.1 While Preserving Baseband 1.59.00. Doing this way you can update to iOS with old baseband (iPhone 4 Only).
So first update to iOS 4.1 while preserving your baseband (instructions here) then jailbreak with limera1n (instructions here) and then follow the following instructions to unlock your iPhone 4 iOS 4.1 baseband with Ultrasn0w.
Instructions to install ultrasn0w using cydia
Step One : Press to launch Cydia Installer from your Springboard.
Step Two : Press to select the Manage tab at the bottom of the screen.
Step Three : Press to select the large Sources button
Step Four : Press the Edit button at the top right of the screen.
Step Five : Press the Add button at the top left of the screen.
Step Six : Enter http://repo666.ultrasn0w.com as the source url and press the Add Source button.
Note:ultrasn0w is spelled with a zero, not the letter “o”.
Step Seven : Once the source has been added press the large Return to Cydia button.
Step Eight : Press the Done button at the top right of the screen.
Step Nine : Press to select http://repo666.ultrasn0w.com/ from the list of user entered sources.
Step Ten : Press to select ultrasn0w from the list of packages.
Step Eleven : Press the Install button at the top right of the screen.
Step Twelve : Press the Confirm button at the top right of the screen.
Step Thirteen : After installation completes successfully press the reboot.
Step Fourteen : Now press the Home button then power off and power on the iPhone. You do this by holding down the power button for 3 seconds then moving the power slider that appears to the right. Press the power button again to turn on the phone.
Step Fifteen : You should now be able to insert the SIM of your choice!
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