The 'Four Class System' was a legal caste system in the Yuan Dynasty. After the founding of the Yuan regime, Kublai Khan, the first emperor in the Yuan Dynasty, set up this system to consolidate the ruling status of the Mongolian Ethnic Minority, which had a small number with great disparity to the majority Han people.
Specifically, the four classes of people by the descending order were Mongolian people, Semu people, Han people (in the northern areas of China) and Southerners (people of the former Northern Song Dynasty (1127 - 1279)). According to the Yuan rulers' mind, the grading sequence was based on the sequencing of Mongolian's conquest of these people. Some historical scholars said that it was a kind of psychological indication that the earlier they submitted to Mongolian people, the higher social status they would be.
Unfairly, the 'Four Class System' stipulated that four classes of people received different treatment in political, legal and military affairs. First, the real power was mainly grasped in the hands of the Mongolian people and Semu people while few of the court officials were Han people or any other ethnic minorities from the third and the fourth classes. Second, although all classes of people were allowed to attend the imperial examination, people of the third and fourth classes had to participate in more test subjects and exam questions that for them were more difficult, compared with the first and second class. Third, the fourth class people received unequal legal treatment. On committing the same crime, different punishments were handed down to different classes of people. Fourth, the Mongolian people adopted a tight control towards the Han people and Southerners. These two classes were forbidden to possess any weapon or raise any dogs or eagles.
Generally, the 'Four Class System' was a national policy of political oppression and ethnic division. Originally, it was established by Yuan's ruling class to guarantee the dominance of the Mongolian minority but it eventually became the catalyst that sped up the decline of the Yuan regime.