The Analects of Confucius is a collection of quotations from Confucius and his disciples. The University is an academic report by Ceng Zi, a disciple of Confucius. The Doctrine of the Mean is a report learned by Confucius' grandson Zisi. Zisi is a student in Ceng Zi. Mencius is a student of Ceng Zi's later generations and a work of the Warring States Period.

So according to the writing time: Analects of Confucius → University → The Doctrine of the Mean → Mencius.

PS: The Great Learning and The Doctrine of the Mean were not independent articles before the Song Dynasty, but two chapters in the Book of Rites. In the Song Dynasty, Zhu, a great scholar, took it out as the second of the four books and usurped the original text of Great Learning privately. So there are two versions of the university.

In ancient times, the order of learning generally started from the university, followed by The Analects of Confucius, The Doctrine of the Mean and Mencius.

Because the four books were written after the Song Dynasty, according to Zhu Zhuben's understanding, the words in universities are relatively simple and clear. (In fact, this is not the case at all. The Analects of Confucius is a basic introduction to Confucianism, which is simple to explain. "The Doctrine of the Mean" and Mencius mainly talk about emperors and the way to use the world, so they are placed at the end. Generally, the four books will be mixed with the five classics. For example, The Book of Songs and The Classic are both studied together, because there are many quotations in the four books.

The above statement is a rough idea, because unlike modern times, there were no government-run schools in ancient times, and they were all private schools, so there was no unified standard for basic education methods. The above can only be used as a reference, not as a theorem.