What do the four schools of thought in the Southern Song Dynasty mean?

Li Tang, Liu Songnian, Ma Yuan and Xia Gui, landscape painters in the Southern Song Dynasty, were also called "Li Liu Ma Xia". Four paintings belong to painting style. There was no such name at the beginning. Dong Tang of the Yuan Dynasty said: "For example, Li Tang, Zhou Zeng, Ma Yuan, Xia Gui, Li Jue, Li Anzhong, Lou Guan and Liang Kai, all those who were named in the Southern Song Dynasty Painting Academy were appreciated by Li Tang, and the rest could not be left alone." The proposer of this theory is Tang Yin's title of Liu Songnian's "The Hidden Map of Spring Mountain Fairy" in Ming Dynasty, which is called "Li Liu Ma Xia" in history. Later, Tu Long said: "Li Tang, Liu Songnian, Ma Yuan, Xia Gui, four people from Nandu." Zhang Chou said: "The top scholar in the Southern Song Dynasty was Liu Songnian, followed by Li Tang, Ma Yuan and Xia Gui." Zhang Taijie said: "Liu, Li, Ma and Xia all have heavy names, but Li and Ma are the most important." In a word, the four schools of painting in the Southern Song Dynasty ruled the landscape painting field in the Southern Song Dynasty for more than one hundred years. They add luster to history and painting style, and celebrities are eternal. For example, Li Tang created the "axe chopping" method in his later years, and his representative works include The Map of Wanhe and Song Feng. Liu Songnian people are called "Liu Qingbo" and "Mi Men Liu", and his masterpiece is Four Scenes. Ma Yuan's painting history calls him "the corner of the horse". Representative works include Walking Map and Water Map. Xia Gui's painting style is similar to that of Xia, which is called "Horse and Summer", and painting history calls it "Xia Banbian". Representative works include "Xishan Qingyuan Map" and so on. They also influenced the landscape paintings of Zhejiang School in Ming Dynasty, even as far away as the East, and were highly valued by Japanese painting circles.