You should use this small hook code in your current active WordPress theme's functions.php file:

add_filter('wpforo_content_after', 'wpforo_hide_links_for_guests', 10);
function wpforo_hide_links_for_guests( $content ){
    if( WPF()->current_user_groupid === 4 ){
        $content = preg_replace('|<a+href=+>+<\/a>|im', '<a href="#" rel="nofollow"></a>', $content);
    }
   return $content;
}

Here is a good article how to add such codes to the website in case you cannot add it in the functions.php file: https://www.wpbeginner.com/plugins/how-to-easily-add-custom-code-in-wordpress-without-breaking-your-site/