Which four calligraphers does Yan refer to?

Wang Xizhi, Yan "refers to Yan Zhenqing, Liu" refers to Liu Gongquan's Yan Gu, which means that their styles are different from each other. Yan Zhenqing's regular script embodies a prosperous style and momentum; Liu Gongquan studied under Yan Zhenqing and absorbed his own new ideas, which made his ci avoid the trend of horizontal and vertical, but adopted a balanced and thin and hard approach, firmly pursuing Wei Bei, subtle and graceful, vigorous and compact. Many scholars in later generations took Liu Zi as a model. Liu Gongquan's regular script involves Ou Yangxun's brushwork, and some strokes are often closely interspersed, which is particularly broad, fine and angular. Although the pen was written by Yan Zhenqing, it is different from Yan Zhenqing's vigorous and broad style, which is particularly heroic. Yan Zhenqing's calligraphy is full of strength and style; Liu Gongquan's calligraphy emphasizes strength and strength, so it is also called "Yan Gu". Yan Zhenqing's cursive script is simple and vigorous, with ups and downs. His pen is magnificent, ingenious and natural, with the flavor of seal script, but the cursive script still does not lose its Wei and Jin demeanor. Song Fan Zhongyan entrusted Shi: "The pen to prolong life, Yan Gu." Yan Zhenqing's words are full of "tendons" and Liu Gongquan's words are full of "bones", so there is a saying of "Yan Liu Jin Gu".