Since the ErrorHandler plugin captures not only application errors, but also errors in the controller chain arising from missing controller classes and/or action methods, it can be used as a 404 handler. To do so, you will need to have your error controller check the exception type.

Exceptions captured are logged in an object registered in the request. To retrieve it, use Zend_Controller_Action::_getParam('error_handler'):

class ErrorController extends Zend_Controller_Action
{
    public function errorAction()
    {
        $errors = $this->_getParam('error_handler');
    }
}

Once you have the error object, you can get the type via $errors->type;. It will be one of the following:

You can then test for either of the first three types, and, if so, indicate a 404 page:

class ErrorController extends Zend_Controller_Action
{
    public function errorAction()
    {
        $errors = $this->_getParam('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ROUTE:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:
                // 404 error -- controller or action not found
                $this->getResponse()
                     ->setRawHeader('HTTP/1.1 404 Not Found');

                // ... get some output to display...
                break;
            default:
                // application error; display error page, but don't
                // change status code
                break;
        }
    }
}

Finally, you can retrieve the exception that triggered the error handler by grabbing the exception property of the error_handler object:

public function errorAction()
{
        $errors = $this->_getParam('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ROUTE:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:
                // 404 error -- controller or action not found
                $this->getResponse()
                     ->setRawHeader('HTTP/1.1 404 Not Found');

                // ... get some output to display...
                break;
            default:
                // application error; display error page, but don't change
                // status code

                // ...

                // Log the exception:
                $exception = $errors->exception;
                $log = new Zend_Log(
                    new Zend_Log_Writer_Stream(
                        '/tmp/applicationException.log'
                    )
                );
                $log->debug($exception->getMessage() . "\n" .
                            $exception->getTraceAsString());
                break;
        }
}

If you dispatch multiple actions in a request, or if your action makes multiple calls to render(), it's possible that the response object already has content stored within it. This can lead to rendering a mixture of expected content and error content.

If you wish to render errors inline in such pages, no changes will be necessary. If you do not wish to render such content, you should clear the response body prior to rendering any views:

$this->getResponse()->clearBody();

In order to use the Error Handler plugin, you need an error controller. Below is a simple example.

class ErrorController extends Zend_Controller_Action
{
    public function errorAction()
    {
        $errors = $this->_getParam('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ROUTE:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:
                // 404 error -- controller or action not found
                $this->getResponse()->setRawHeader('HTTP/1.1 404 Not Found');

                $content =<<<EOH
<h1>Error!</h1>
<p>The page you requested was not found.</p>
EOH;
                break;
            default:
                // application error
                $content =<<<EOH
<h1>Error!</h1>
<p>An unexpected error occurred. Please try again later.</p>
EOH;
                break;
        }

        // Clear previous content
        $this->getResponse()->clearBody();

        $this->view->content = $content;
    }
}