function serve_404_for_specific_post_types($template) {
    if (is_singular(array('client', 'testimonial'))) { /* here is add posttype slug */ 
        global $wp_query;
        $wp_query->set_404();
        status_header(404);
        // You can either load your theme's 404 template or specify a custom 404 template
        return get_404_template();
    }
    return $template;
}
add_filter('template_include', 'serve_404_for_specific_post_types');