ImpressCMS 1.4 is a intermediate release between the legacy 1.3 series and the upcoming 2.0 release. The focus is on providing compatibility with PHP 7 and code cleanup. Existing modules that are compatible with PHP 7 should still work, as long as they are compatible with PHP7 and the obligatory PDO database driver that switching to PHP7 brings with it.
Unzip the contents of the 'htdocs' folder in the root folder of your site, and go in a browser to the address of your site. ImpressCMS will detect that no configuration is present, and will start the installation procedure to help you setup your site on your webserver.
When updating from a previous version in the 1.3 series, or from a previous 1.4 release, overwrite your existing installation and update the system module once you are logged in, that should bring you to the latest version. When upgrading your PHP, please verify that the modules you use are compatible!
None so far. If you encounter any problems, please log a ticket at https://github.com/ImpressCMS/impresscms/issues/new/choose