Because 4 is most likely the devil's number. Do you often see something good associated with this number? And the bad one? So I don't either. But how to explain the fact that a cancerous tumor needs exactly four stages to kill a person? This is probably inexplicable, but still a fact.
This classification was invented for convenience in 1956. Stage I - a limited tumor process (up to 2 cm) without affecting the nearest lymph nodes; Stage II - a mobile tumor (from 2 cm), a single mobile metastasis in the nearest lymph nodes; Stage III - the tumor is limited in mobility, metastases in the nearest (regional) lymph nodes are determined; Stage IV - a tumor of any size with distant metastases or growing into adjacent organs.