I had 4th version, too, and also have faced the troubles with activating on Windows 7 (error ‘You have entered an incorrect registration code’). After lots of unsuccessful tries, I was able to make everything work, and that’s how:
* Uninstalled the version which I was not able to activate manually (that is before installing again);
* Disabled antivirus and firewall;
* Installed via “run as admin”;
* Re-enabled antivirus and firewall.