Try this in your .htaccess:
.htaccess
ErrorDocument 404 http://example.com/404/
ErrorDocument 500 http://example.com/500/
# or map them to one error document:
# ErrorDocument 404 /pages/errors/error_redirect.php
# ErrorDocument 500 /pages/errors/error_redirect.php
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} ^/404/$
RewriteRule ^(.*)$ /pages/errors/404.php [L]
RewriteCond %{REQUEST_URI} ^/500/$
RewriteRule ^(.*)$ /pages/errors/500.php [L]
# or map them to one error document:
#RewriteCond %{REQUEST_URI} ^/404/$ [OR]
#RewriteCond %{REQUEST_URI} ^/500/$
#RewriteRule ^(.*)$ /pages/errors/error_redirect.php [L]
The ErrorDocument redirects all 404s to a specific URL, all 500s to another url (replace with your domain).
The Rewrite rules map that URL to your actual 404.php script. The RewriteCond regular expressions can be made more generic if you want, but I think you have to explicitly define all ErrorDocument codes you want to override.
Local Redirect:
Change .htaccess ErrorDocument to a file that exists (must exist, or you'll get an error):
ErrorDocument 404 /pages/errors/404_redirect.php
404_redirect.php
<?php
header('Location: /404/');
exit;
?>
Redirect based on error number
Looks like you'll need to specify an ErrorDocument line in .htaccess for every error you want to redirect (see: Apache ErrorDocument and Apache Custom Error). The .htaccess example above has multiple examples in it. You can use the following as the generic redirect script to replace 404_redirect.php above.
error_redirect.php
<?php
$error_url = $_SERVER["REDIRECT_STATUS"] . '/';
$error_path = $error_url . '.php';
if ( ! file_exists($error_path)) {
// this is the default error if a specific error page is not found
$error_url = '404/';
}
header('Location: ' . $error_url);
exit;
?>