WordPress `throw_404()` helper.

functions-404.php

<?php

/**
 * Throw a 404 error and render the current theme's 404 template.
 *
 * @see https://richjenks.com/wordpress-throw-404/
 * @see https://developer.wordpress.org/reference/hooks/pre_get_document_title/
 */
function throw_404() {
	// 1. Ensure `is_*` functions work.
	global $wp_query;
	$wp_query->set_404();

	// 2. Reset HTML title.
  // NOTE: 👇 Change this to customize your site's 404 title.
	add_filter( 'pre_get_document_title', function () {
		return 'Page Not Found';
	}, 999 );

	// 3. Return a 404 status code.
	status_header( 404 );
	nocache_headers();

	// 4. Show 404 template.
	require get_404_template();

	// 5. Stop execution.
	exit;
}