sfPHPCaptchaPlugin is not stable for symfony 1.4 as it said in the plugin information Here.
You can use sfCaptchaGDPlugin its a very good one, folow the intalation guide in README, and use this widget:
$this->widgetSchema['captcha'] = new sfWidgetCaptchaGD();
$this->validatorSchema['captcha'] = new sfCaptchaGDValidator(array('length' => 4));