在破解了一段时间后实现了自己的愿望

Fished my own wish after hacking on it for a while

$now = Mage::getSingleton('core/date')->timestamp( time() );
$websiteId = Mage::app()->getStore()->getWebsiteId();
$customerGroup = 4;

Mage::getResourceModel('catalogrule/rule')->getRulePrice( $now, $websiteId, $customerGroup, $_productId);