The iPod 'iPod' could not be restored. This device isn't eligible for the requested build.?

I have a second generation iPod Touch that I bought about a year ago. Yesterday I upgraded from the original settings to the new 4.2.1. I want to downgrade back to 4.0 - I tried clicking restore but it didn't do anything, so I researched and download the 'iPod2,1_4.0_8A293_Restore.ipsw' file. I followed instructions and in Recovery Mode held shift when clicking Restore, and selected this file. When it is verifying with Apple, it comes up with the message 'The iPod 'iPod' could not be restored. The device isn't eligible for the requested build.' Can anyone help me? I really want to downgrade back to what I had. Sorry for the lengthy explanation, any help would be appreciated :)