I'm looking for a way to downgrade my phpMyAdmin from version 4.0.5 to let say 3.5 or 3.6 or sth. like that. Does anyone know how to do it? Is it possible without losing all my databases.
It's just a matter of installing version 3.5.8.2. Refer to http://docs.phpmyadmin.net/en/latest/setup.html#quick-install.
If you have not installed phpMyAdmin configuration storage tables that are specific to version 4, you have nothing else to do. Of course, it depends on the URL you are using to visit phpMyAdmin, and whether you installed version 4.0 by yourself or used a prepackaged installation.
I don't see why you would lose your databases by doing so.