@justforthelulz, A short one, 1 or 2 words, is more preferable since the characters will use it when talking.
@TjapDjemboet, So far in the story, they only talk about one 闇の王 (King of Darkness). And there's no reference to any 4 bosses or enemies.
Btw in general, フォー refers to: four, for, foe...