Originally Posted by
Olethros
You haven't got your hosts file modified to either talk to TinyUmbrella's TSS server or to Saurik's server. By default, iTunes will talk to an Apple server (gs.apple.com) during restore and request a SHSH. The real gs.apple.com will reject a request for a 4.1 SHSH.
Simplest solution, download TinyUmbrella, go to the advanced tab. Make sure "set hosts to Cydia on exit" is ticked. Then click apply changes and close Tiny Umbrella.
Try to restore the custom IPSW (with the phone in pwned DFU mode - not just normal DFU mode) again and it should work