In Japanese folklore, ayakashi refers to any supernatural entity . . . that “appears above the surface of some body of water.” In many stories, they are “the vengeful spirits of those who died at sea,” but in others, they are simply atmospheric ghost lights, a phenomenon often associated with UFO sightings . . .
Given there’s nothing remotely nautical or maritime about the Four Sisters, it seems doubtful [Naoko] intended to reference vengeful sea ghosts with her use of ayakashi. With UFOs, crop circles, and related phenomena at the center of the Black Moon Clan arc, it seems far more likely she was referring instead to atmospheric ghost lights.
For this reason I think the best way to translate Ayakashi no Yon Shimai would be with a word that evokes that connection, that evokes the experience of witnessing that strange and mysterious phenomenon. “The Four Eerie Sisters” is probably the best option, with “Uncanny,” “Unearthly,” and “Otherworldly” being good possibilities as well.