Those who are new to this, iFaith tool created by iH8sn0w developer which not only allows you to Save SHSH blobs but also produce signed IPSW. Althought NotCom’s TinyUmbrella utility also enables you to save SHSH blobs but it doesn’t have the ability to produce signed IPSW. But if you are not satisfied with new iOS upgrade then these saved blobs will used to downgrade your iDevice iOS to an previous or older version. It means uptill now, if you haven’t seen saved your SHSH blobs before upgrading to new firmware then you may not able downgrade it to older firmware.
Now a group of hackers named themselves The Private Dev Team have created a new utility called Bluefreeze which allows you to downgrade (tethered only) to any iOS firmware version without having saved SHSH blobs. Hackers have managed to do it by modifying the firmware version (and firmware checksum) in the iFaith certificate file which makes this check disabled. Thus firmware will be installed without signatures.
Bluefreeze asks you to build and browse to two ipsw’s one signed properly and one not signed. Then Bluefreeze swaps the properly signed img3 files in the properly signed firmware file with the incorrectly signed img3 files in the unsigned ipsw thus resulting in an ipsw file with properly signed img3 files. This firmware file is used for the downgrade.
Having an incorrectly signed firmware installed won’t let you boot of course. But because the limera1n exploit ignores incorrect signitures we can use the limera1n exploit (DFU mode, then using redsn0w) to boot up your device. The problem is only that you have to repeat this every time (similar to a tethered jailbreak), so it’s not a downgrade you would want. This should be your last resort, and only if you absolutely need a downgrade.
iPhone 4 (CDMA / GSM), iPhone 3GS, iPod Touch 3G, iPod Touch 4, Apple TV 2G, iPad 1G. However, A5 chip driven devices which includes iPhone 4S and iPad 2 are not supported.
You can downgrade your above mentioned A4 device to iOS 4.3, 4.3.5, 4.2.1, 4.1 or 5.0 from 5.0.1 firmwares.