The iPhone 4 can be downgraded to any firmware if you have the valid SHSH and aptickets. It might seem like a very simple thing to do, but the information on how to do it is scattered around all over the web and all of the below steps are necessary.
Steps[edit source]
Build a signed IPSW using iFaith[edit source]
1. Download iFaith from http://ih8sn0w.com
Note: This step can be performed on windows in VMware
2. Get your SHSH blobs ready. Tinyumbrella stores the SHSH blobs in %userprofile%\.shsh or ~/.shsh/
3. Build your firmware with iFaith and the blobs
Downgrade iTunes to 11.0.5[edit source]
Since iTunes 11.1 Apple prevents downgrades, therefore you have to downgrade iTunes to 11.0.5. More details can be found here: http://blog.ih8sn0w.com/2013/09/some-updates.html
1. You can download iTunes 11.0.5 from:
Windows (32 Bit): https://api.ipsw.me/v2.1/iTunes/win/11.0.5/url/dl
Windows (64 Bit): https://api.ipsw.me/v2.1/iTunes/win/11.0.5/64biturl/dl
OS X: https://api.ipsw.me/v2.1/iTunes/osx/11.0.5/url/dl
2. Installation
Windows:
1. Use Control Panel to uninstall iTunes
2. Run the downloaded installer to install iTunes
OS X:
Use pacifist (http://www.charlessoft.com) to install the old version (Default installer won't let you)
Put your iPhone 4 into pwned DFU mode[edit source]
Windows: Use iFaith to put your iPhone 4 into DFU Mode
OS X: To actually restore the signed IPSW your iPhone 4 has to be in the pwned DFU mode. The version of redsn0w I have been trying to use did not work, therefore I recommend using ih8sn0w's iReb: http://ih8sn0w.com
It is available for OS X and windows.
Note: This step cannot be performed in VMWare
Restore the signed IPSW[edit source]
Windows: Once your iPhone 4 is in pwned DFU mode, simply start iTunes and shift-click the restore button. Select the signed IPSW and it should work.
OS X: Once your iPhone 4 is in pwned DFU mode, simply start iTunes and alt-click the restore button. Select the signed IPSW and it should work.
Note: This step cannot be performed in VMWare