What is the concept of the holy king in the four sentences of Hengqu?

Two different things. You may remember wrong.

1, Hengqu four sentences: that is, "stand for heaven and earth, stand for the people, link the past with the future, and be peaceful for all generations", which is the famous saying of Zhang Zai, a Confucian scholar in the Northern Song Dynasty. Feng Youlan, a contemporary philosopher, called it "four sentences in a horizontal canal". Because of its conciseness, it has always been praised by people. It means that a scholar's heart should represent the world, and his life should represent all people. He wants to inherit and carry forward the unique study of the sage, create a peaceful foundation for all generations, and tell the aspirations and pursuits that a scholar should have: the world, all peoples, the way of sages, and a peaceful foundation.

2. The Inner Sage and the Outer King: First, from Zhuangzi Tianxia written by Zhuang Zhou in the pre-Qin Dynasty: "The inner sage and the outer king are dark and unknown, depressed and do nothing, and everyone in the world does whatever they want, taking themselves as their own, which is the Tao." This was originally elaborated by Taoism and used by Confucianism. "Inner Sage" means self-cultivation, moral cultivation, and requires people to be virtuous; The "foreign king" means managing the house, governing the country and leveling the world. The unity of "inner sage and outer king" is the highest realm pursued by Confucian scholars. This sentence applies to both countries and individuals. A simple understanding is that others will be convinced when they are strong (this strength does not just refer to strength, but to comprehensive national strength or comprehensive quality).