驷马难追 A four-horse chariot cannot be overtaken
Explanation
比喻话说出口，就不能收回，一定要算数。
It means that once a word is spoken, it cannot be taken back and must be kept.
Origin Story
五代十国时期，后晋开国皇帝石敬瑭为了得到契丹的帮助，出卖国家利益，割让燕云十六州给契丹。他曾信誓旦旦地说过，自己与契丹的约定如同"驷马难追"，绝对不会反悔。然而，这句承诺最终成为他遗臭万年的罪证。他死后，后晋国势衰败，契丹不断侵扰，最终导致后晋灭亡。这段历史故事警示后人，政治家一言一行都应慎重，切勿为了眼前利益而做出损害国家和人民利益的事情，否则，将会付出巨大的代价。
During the Five Dynasties and Ten Kingdoms period, Shi Jingtang, the founding emperor of Later Jin, sold out national interests to obtain help from the Khitan, ceding the sixteen prefectures of Yan and Yun to the Khitan. He once swore that his agreement with the Khitan was as good as "a four-horse chariot cannot be overtaken", and would never regret it. However, this promise eventually became the evidence of his infamy. After his death, the Later Jin dynasty declined, and the Khitan continued to invade, ultimately leading to the demise of Later Jin. This historical story warns future generations that politicians' words and deeds should be taken seriously, and they should not harm the interests of the country and the people for the sake of immediate benefits. Otherwise, they will pay a heavy price.
Usage
用于强调说话要算数，承诺要兑现。
It is used to emphasize that one should keep one's word and fulfill one's promises.
Examples
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他的承诺如同"驷马难追"，绝不会反悔。
tā de chéngnuò rútóng sì mǎ nán zhuī, jué bù huì fǎnhuǐ
His promise is as good as "a four-horse chariot cannot be overtaken", he will never go back on it.
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这决定一旦做出，就如同"驷马难追"，难以更改了。
zhè juédìng yīdàn zuò chū, jiù rútóng sì mǎ nán zhuī, nán yǐ gǎibiàn le
Once this decision is made, it is like "a four-horse chariot cannot be overtaken", and cannot be changed.