Question: How to set up redirect of non-existent pages to 404 error page? 

Answer: Go to ISPmanager control panel, then “World Wide Web” menu – “Error pages” and click “create”. In the appeared menu, please select the domain needed and error code. In URL field, indicate a link to the previously prepared page with 404 error. This page will be always displayed when a users try to visit a non-exostent page on your site.