Who are "Yan, Liu, Ou and Zhao" among ancient calligraphers?

Yan Zhenqing (709-785) was an outstanding calligrapher in the Tang Dynasty.

Liu Gongquan (778-865) was the last great calligrapher in the Tang Dynasty.

Ou Yangxun (557-64 1 year), Han nationality, was born in Linxiang, Tanzhou (now Changsha, Hunan), and was one of the four masters of regular script (Ou Yangxun, Yan Zhenqing, Liu Gongquan and Zhao Mengfu).

Zhao Mengfu (1254— 1322), a famous painter in Yuan Dynasty, was one of the four masters of regular script (Ou Yangxun, Yan Zhenqing, Liu Gongquan and Zhao Mengfu).