Some previous work around when jailbreaking Firmware 4.1. is not causing error to some greensn0w 4.2.1 jailbreakers. This was due to the “hosts” being modified. Follow instructions below for the fix.

Warning: This will result in your iPhone 4 baseband being upgraded to the latest version.

for Mac OS X:

1. Hit command+space (open spotlight)
2. Type in Terminal (select the one from the Applications/Utilities folder
3. Type “sudo nano /etc/hosts” (without quotes) and hit return.
4. Enter your password
5. Use the down arrow key to find the “gs.apple.com” entries. Once the cursor is in front, make sure you comment out the line(s) by entering “#” in front of the text. (to disable that line or comment it out)
6. Save the file by pressing control+O (then enter to accept).
7. Exit the nano editor by pressing control+X.
8. Now your done. Try restoring again your device.

for Windows:

1. Win+R (open “Run”)
2. Make sure your in “Administrator” mode or UAC disabled so you can edit your hosts file
3. type in “notepad c:\Windows\System32\drivers\etc\hosts” (without the quotes, this is if your windows is in drive “C:”).
4. Use the down arrow key to find the “gs.apple.com” entries. Once the cursor is in front, make sure you comment out the line(s) by entering “#” in front of the text. (to disable that line or comment it out)
5. Hit Save
6. Now your done. Try restoring again your device.