Below is the detailed steps to restore iOS4.0.1 on iPhone 3GS lock version.
If you want to restore your iPhone again to iOS 4.0.1.You wont be able to restore your iPhone 4 on iOS 4.0.1 by simply pointing iTunes to iOS 4.0.1 ipsw file because Apple has stopped that.Further you do so iTunes will throw an error: “This iPhone [your name] iPhone could not be restored. This device isn’t eligible for the requested build.
To fix this error you have to change your system’s “host” file. Use Cydia server instead of Apple’s.One error may be seen again such as:1004, or 1015 from iTunes saying: “The iPhone [you name] iPhone could not be restored. An unknown error occurred (1004)”.
You may try to fix these errors by following the steps thats are given below.
Step 1: Download the required iOS 4.0.1 ipsw firmware files. download URL
Step 2: Put your device into DFU mode (not recovery mode).
* Connect your iPhone to your computer.
* Turn your 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 3: Before you proceed, you must make sure that you have your SHSH blobs saved on Cydia for the firmware version you restoring or downgrading to. If you don’t have your SHSH files saved, you cannot downgrade or restore to iOS 4.0.1.
So now, what we will do is to trick iTunes into thinking that it is watching Apple servers, while in real, we are going to change the address to Saurik’s server.
* Navigate to C:\Windows\System32\drivers\etc\ and locate “hosts” file.
Mac OS X users can get to “/etc/” directory by going through “Go –> Go to Folder” from Finder.
* Open this “hosts” file in Notepad (for Windows) or TextEdit (for Mac OS X) and add this
Now simply save this file.
Step 4: You will now have to restore your iPhone to the firmware you downloaded in Step 1. Click on your phone icon from the sidebar in iTunes. Now press and hold left “alt” (option) button on Mac (Left “Shift” button on Windows) on the keyboard and then click on “Restore” (Not “Update” or “Check for Update”) button in the iTunes and then release this button.
This will make iTunes prompt you to select the location for your downloaded firmware file. Select that ipsw file and click on “Open”.
Step 5: Now sit back and enjoy as iTunes does the rest for you. This will involve a series of automated steps. Be patient at this stage and don’t do anything silly. Just wait while iTunes installs firmware on your iPhone. Your iPhone screen at this point will be showing a progress bar indicating installation progress. Now at some point, iTunes will probably throw 1004 or 1015 error at you. This is normal, just ignore this error as the firmware has already been successfully installed on the device.
Step 6: Your iPhone at this point will be stuck in Recovery Mode, showing that “Connect to iTunes” screen. To exit your device from Recovery Mode, download this program called TinyUmbrella (Windows / Mac). Run it and the click on “Kick Device Out of Recovery” button. Your device will now restart normally.
Since my iPhone 3GS is locked, so i have to use my original AT&T SIM card to active it again and then use ultrasn0w to unlock it.