    $form = $this->add('Form');
    $form->addField('Line','captcha')->add('x_captcha/Controller_Captcha');
    $form->addSubmit('Check');
    $form->onSubmit(function($form){
        if ($form->getElement('captcha')->captcha->isSame($form->get('captcha'))) {
            $form->js()->univ()->successMessage('Captcha is OK!')->execute();
        } else {
            $form->js()->atk4_form('fieldError','captcha','Wrong captcha!')->execute();
        }
    });

If you need case unsensetive captcha just pass false as a second argument for method isSame

    if ($form->getElement('captcha')->captcha->isSame($form->get('captcha',false)))