For pages no longer used or linked in our site is it better to delete them entirely from our server and return a 404 error or leave them as orphaned pages in case they have been indexed by the search engines? There are no alternative pages to do a 301 redirect.
I'd vote neither. When a page is no longer needed and there's no sound historical benefit to keeping it online, I delete the page and set the htaccess to return a 410 error code (gone, not coming back) and eradicate any internal links to the page.
Any external link bringing someone to that page will result in a custom error page which includes a one line explanation and a link back to home page of the site.