Which four people did Mi Cai, a great calligrapher in the Northern Song Dynasty, refer to?

The calligrapher "Mi Cai" in the Northern Song Dynasty usually refers to Su Shi, Huang Tingjian, Mi Fei and Cai Xiang. Briefly as follows:

1, Su Shi's calligraphy is ups and downs, naive;

2. Huang Tingjian's calligraphy is vertical and horizontal;

3. Mi Fei's calligraphy is unrestrained, calm and happy;

4. Cai Xiang's calligraphy adopts the method of Jin and Tang Dynasties, paying attention to the ancient meaning and statutes.