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Downgrade iPhone 4, 3GS from iOS 4.0.2 to 4.0.1 Without SHSH Blobs Saved
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The step by step instructions for downgrading new iPhone 4 and iPhone 3GS without having SHSH blobs saved are posted below. Follow them at your own risk. I am not responsible for any loss of data, or malfunction of your iPhone.

Step 1: Download iOS 4.0.1 ipsw.

Step 2: Extract it with WinRAR or WinZip to a folder on the desktop. You may need to rename the firmware file from .ipsw to .zip to do this.

Step 3: Open the buildmanifest.plist with the Notepad if you are on Windows, or TextWrangler if you are on Mac.

Search and replace all – 8A306 with 8A400. Save. Repeat the same with the file restore.plist.

Step 4: Download iOS 4.0.2 ipsw and open this with WinRAR or WinZip.

Note: Do not extract it. Just open it and leave it open. You must use this exact file and not create a new one. If you have to create a new one for reasons like you are on OS X, then use zip command line not explorer or finder to make the zip. I will assume you are using the original file opened in WinRAR for the rest of this guide.

Step 5: Take all the files from iOS 4.0.1 and drag them over to the iOS 4.0.2 zip archive that you have open.

Step 6: Delete all the .dmg files that have 002 at the end, leaving only the 001 files left.

Step 7: Save the archive, and rename it back to .ipsw if you changed the name to get WinRAR/WinZip to open it.

Step 8: Optional (This helps ensure you get an SHSH file request for the future, but should not be necessary to just restore iOS 4.0.1).
Add the 74.208.10.249 gs.apple.com line to the host file. If you need help with this step, read Step 3.

Step 9: Put the device in DFU by following the steps below:

* 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 iPhone.

Note: Your iPhone screen at this time should be blank (black in color), if not, then you are most likely in Recovery Mode, not DFU mode.

Step 10: Now simply open iTunes and restore the firmware you changed.

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