You have to have a 4.0 OS somewhere on your computer then if its telling you to downgrade...

Usually what happens, is the OS that you have extracted onto your computer is LOWER than the OS that is on your device.

It should give you an option whether you want to keep the current operating system on your device (Recommended) or downgrade to the lower version of the Operating System...

So somehow it looks as if you installed 4.0 onto your computer and extracted it, otherwise the Desktop Manger wouldn't give you that message.