Not much is known about the origins of the Four Kings, except a single obscure tale told in certain noble families. The tale describes four kings, each ruling a powerful empire that fell into war and infighting by the influence of a mysterious entity due to fear and ignorance.
Apparently, they did exist, as the Margasters utilized the souls of their sacrifices to summon the shades of the Four Kings into the real world. Fortunately, those were defeated by the Seekers and their allies.