It's actually quite simple. The civilizations of ancient India, Egypt and China were all based on the state, and the civilization of the two river basins refers to the small city-states formed in the two river basins. When it was civilized, it was still a city-state, and the unit was a city, while Babylon was a country after the unification of the city-states in two river basins, so the four major civilizations were: ancient Egypt, ancient India, ancient China and ancient river basins, and four ancient countries.