This upgrade breaks theme compatibility with previous releases, including RC2. This means that themes will need to be rebuilt once the upgrade is performed, especially the HEADER and GLOBAL templates, and the CSS. This compatibility break was required to bring forward basic compatibility with IE8 Beta 2, and Google Chrome, as well as to fix many usability issues in default styles.
To upgrade from any previous 4.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").
Please make sure you take a backup before uploading your new files!
» 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.