This isn't a question but a solution that I found to an issue that I hope will help others. I couldn't find a discussion board on this so I'm creating a new one.
I started getting the error message, "Error: Check Activation" when I was trying to download ebooks on my Windows 8 PC. I noticed this was problem for 2.0 as well, and when I tried to erase authorization and reauthorize (which seemed to be the way to fix the problem) it wouldn't allow me to. Then I tried to uninstall and reinstall which didn't work. Then I found the page for if you were having trouble installing ADE. I didn't really have problems with that part but I figured I would give it a try.
One this page, it says to "manually" install the program (which involves downloading the actual zip file and going through that). I did this and it fixed everything! I was able to erase the authorization and reauthorize after this.
I figured I would put this out there so that others might find it useful. I don't know this is specifically a Windows 8 problem or a v4.0 problem or what, but I would suggest trying to "manually" install the program before doing any of the other super complicated fixes that I came across and was scared to try.