A way to get this to work on 4.0 Rated 5 out of 5 stars

For windows users use the run program (Start>Run or Windows Key+R) then type in %appdata%

After that navigate to \Roaming\Mozilla\Firefox\Profiles\*profilenamehere*\extensions\redshift_V2@shift-themes.com

You will see a file called install.rdf All you have to do is open that in notepad, Change this:
<em:minVersion>3.5</em:minVersion>
<em:maxVersion>3.6.*</em:maxVersion>
To this : <em:minVersion>3.5</em:minVersion>
<em:maxVersion>4.*</em:maxVersion>

Note that not everything will look right, but firefox will run like normal and you'll have a nice looking theme