How To Downgrade iOS 5.1 to 4.3.5 And Below

*Note: This does not work for iPhone 4S
Step 1: Download the iPSW of the firmware version you would like to revert to, as listed below here :

• iOS 4.3.5
• iOS 4.3.4
• iOS 4.3.3

Step 2: Connect your device to your PC or Mac, and slip it into pwned DFU mode using the latest version of Redsn0w (look for Pwned DFU option in “Extras”).
Step 3: Next, you’ll need to edit your hosts file. It can be found in Windows > System 32 > drivers > etc on Windows or just etc on OS X. Back it up somewhere safe, and then open it with a text editor such as Notepad or TextEdit.
Step 4: Add the following the following string to the bottom of the file, refer to the screenshot for further assistance:

74.208.105.171 gs.apple.com

Step 5: Open iTunes and select your device on the left pane. Hold down Shift (Win) or Alt (OS X), click Restore and locate your elected iOS 4.x iPSW from the first step.
Step 6: Ignore Error 10xx if it pops up – it’s to be expected. Use TinyUmbrella or Fix Recovery to kick it out of recovery mode.