This is about the four kings from outside Canaan, who conducted a military expedition into the cities of the plain, that is to say, Sodom and Gomorrah, Zoar and so on. These five city states of the plain were ruled by their overlords, these four kings, or at least one leader among them, mentioned in verse 1.
Zoar was the name given to Bela following Lot’s pleading to be allowed to go there when Sodom was to be destroyed by the Lord, and referring to it as a little one (Zoar means small or little) (Genesis 19:20).