if(isset($form_state['values']['step_4']['user'])){
    $uname = $form_state['values']['step_4']['user'];

  }

  if(isset($form_state['values']['step_4']['password'])){
    $password = $form_state['values']['step_4']['password'];
  }

  if (($uname != '') && ($password != '')) {
    if($uid = user_authenticate($uname, $password)) {
      global $user;
      $user = user_load($uid);
      $arr = array ('name'=>$uname,'pass'=>$password);
      user_login_finalize($arr);
    }
    else form_set_error('step_4', t('Wrong Username'));
  }