To upgrade from any previous 4.0.x release, follow the steps in your website's http://mybaseurl/upgrader.php script. You will need to copy the URL of the attached TAR file as one of these steps ("3: Upload an upgrade TAR for extraction").
» alternatively (but not recommended), you can manually upload the files inside the attached TAR file over existing files (7-zip is just one of many tools for tar files). This is not recommended because it will bypass the upgraders addon-aware extraction algorithm and potentially cause problems.
If you don't follow our advice by taking and testing a backup, and end up needing emergency repair help, ocProducts can help charged at our hourly rate.
For more details, see our upgrading tutorial.
Fixes have been made for all known issues that were in 4.0.1.