Wordpress - How to force a 404 on WordPress

You could try the Wordpress function status_header() to add the HTTP/1.1 404 Not Found header;

So your Code 2 example would be:

function rr_404_my_event() {
  global $post;
  if ( is_singular( 'event' ) && !rr_event_should_be_available( $post->ID ) ) {
    global $wp_query;
    $wp_query->set_404();
    status_header(404);
  }
}
add_action( 'wp', 'rr_404_my_event' );

This function is for example used in this part:

function handle_404() {
    ...cut...
    // Guess it's time to 404.
    $wp_query->set_404();
    status_header( 404 );
    nocache_headers();
    ...cut...
}

from the wp class in /wp-includes/class-wp.php.

So try using this modified Code 2 example in addition to your template_include code.

This code worked for me:

add_action( 'wp', 'force_404' );
function force_404() {
    global $wp_query; //$posts (if required)
    if(is_page()){ // your condition
        status_header( 404 );
        nocache_headers();
        include( get_query_template( '404' ) );
        die();
    }
}

I wouldn't recommend forcing a 404.

If you're worried about search engines why not just do a "no-index,no-follow" meta on those pages and block it with robots.txt?

This may be a better way to block the content from being viewed

add_filter( 'template_include', 'nifty_block_content', 99 );

function nifty_block_content( $template ) {
  if ( is_singular( 'event' ) && !rr_event_should_be_available( $post->ID ) ) {
        $template = locate_template( array( 'nifty-block-content.php' ) );
     }
    return $template;
}

You could probably also use this method to load 404.php but I feel that using a page template might be a better option.

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