@DarkFlame
It’s due to a literal translation of what I would assume to be Shiitenou or something like that. A less literal translation, but something that encapsulates what is meant, would be Pokemon’s Elite Four, which use the same heavenly King’s moniker.
I don’t know much on this side of mythology, but it’s origin is supposed to be Buddha’s 4 god kings that command armies and fight against evil or something. A western equivalent would be a a villain calling his 4 butchers the 4 Horsemen of the Apocalypse.