小三终究会被小四替代
[xiăo sān zhōng jiū huì bèi xiăo sì tì dài]
In the social context where there are issues like having mistresses, '小三' refers to the third party breaking into couples illegally, while '小四' will do similarly replacing '小三'. It shows a cynical view of such unstable relationships or statuses, suggesting no one can firmly stay in an unethical position long.