my dear brothers i did many sets downgraded from os 4.0 to 3.1.3 even if u don't have shsh blob & old or new boot rom. now u just have to do the folowing simple steps
Step 1: Download & install libusb-win32-filter-bin-0.1.12.1.zip
Step 2:
Navigate to C:\Windows\System32\drivers\etc\ and locate “hosts” file.
Step 3: Open this “hosts” file in Notepad and add this line in the end
74.208.105.171 gs.apple.com
Step 4: Restart ur PC (its important)
Step 5: Now restore your device by iTune 9.2 (by DFU mode)
Step 6: When u get error No problem just close iTune
Step 7: Now use RecBoot-1.zip to exit recovery mode
Step 8: Your device is normal now use original sim or onesim to activate and use spirit to jailbreak etc
download RecBoot from here http://www.hack2learn.org/wp-content/uploads/downloads/2010/05/RecBoot-Version-1.0-Windows.zip