You can never update or restore to 4.2.1 if you dont have shsh blobs saved for that firmware.
Where you ever JB and used cydia to start saving your blobs?
Did you ever use Tinyumbrella by any chance?
If you're not sure go to TU's site and download and run the latest version.
http://thefirmwareumbrella.blogspot.com/
Then click save SHSH and it will tell you on the screen what blobs are available for your device.
If all you're getting is 4.3 then that's all you can restore or update to.