Have you jailbroken your iPhone 4 yet? Of course, yes! Geohot has released his Limera1n tool yesterday and it works with iPhone 4 very well (at least for me). If you have iPhone 4, you can upgrade to iOS 4.1 without updating the baseband then unlock it using Ultrasn0w. In case you already upgraded to iOS 4.1 while preserving your baseband, you will not be able to use this guide, just wait for a new Ultrasn0w update.
How to upgrade to iOS 4.1 while preserving your baseband 01.59.00
STEP 2: Close iTunes & connect your iPhone 4 to your PC then launch TinyUmbrella and select “Run as administrator”. If you got a Windows Security Alert, check both boxes and click on “Allow Access”.
STEP 3: Hit “Start TSS Server”. While the TSS Server is running well, TinyUmbrella will kill iTunes.
STEP 4: Now launch iTunes and click on the iPhone icon at the left sidebar of iTunes then hold left “alt” button (“Shift” button on Windows) + Shift from the keyboard. Point it to the iPhone 4.1 firmware you already download.
STEP 5: iTunes will go in a bunch of steps to restore your device to iOS 4.1, it should give you error 1004 which indicates that the baseband is preserved successfully.
STEP 6: Get back to TinyUmbrella and hit “Kick Device Out of Recovery” to get your device out of Recovery Mode. let’s unlock!
How to unlock iPhone 4 on iOS 4.1 baseband 01.59.00 with Ultrasn0w
STEP 7: First, you will need to jailbreak your device using Limera1n jailbreak tool from Geohot by following the step by step guide posted here.
STEP 8: After jailbreaking, open Cydia and touch on Manage >> Sources >> Edit and hit the Add button on the top. Type http://repo666.ultrasn0w.com then hit “Add Source”.
STEP 9: After installing the repo, go back to Cydia and search for Ultrasn0w, tap on it and it will be installed automatically.
Well done, now you got iPhone 4 totally jailbroken & unlocked on baseband01.59.00, congrats. We will be happy to hear your questions about any issues or problems facing you while following the guide.