I found a fix online for this. I had errors trying to install 4.2.6 too (error 1013). Try following these steps, I worded them easier to follow than what I had found originially:
1: Open Terminal
2: Copy and paste this - "sudo nano /etc/hosts" without the quotes
3: Press Enter and type in your password
4: Use the down arrow key to get to the IP address looking things. For me there were 3, the top and bottom ones had a "#" in front of it, and the middle did not.
5: On the one without the "#", add one in front like the others
6: After you have added the "#" in front the line that did not have it, press 'Control+O'. Then hit Enter
7: Press 'Control+X'
8: Try Restoring your iPhone. It will install the update and should work now
1: Go to Start -> Programs -> Accessories
2: Run Notepad or Wordpad
3: Click open and browse to Windows/System32/drivers/etc
4: In "Files of Type:" select all documents
5: Open hosts
6: Delete all line that has "gs.apple.com" or something similar to that
7: Hit save
8: Restore your device
That should (will) fix the error. I have done the Mac way and it has worked.