View file Web Template Ktpl/package/system/controllers/ktpl/hooks/error_404.php

<?php

class onKtplError404 extends cmsAction {

    public function run($data){

    	if($this->options['robots'] && $data == 'robots.txt'){

			echo $this->options['robots'];
			$this->halt();

		}
		
		if($this->options['error404']){

			if(ob_get_length()) { ob_end_clean(); }

	        header("HTTP/1.0 404 Not Found");
	        header("HTTP/1.1 404 Not Found");
	        header("Status: 404 Not Found");

	        cmsTemplate::getInstance()->renderAsset('errors/skins/' . $this->options['error404'] . '/index');
	        die();

		}

		return $data;

    }

}