From Dark Horse:
"Birthday," the fourth in a series of five Ring manga, is a trilogy of stories, each enriching the main storyline of the Ring series. "The Casket Floating in the Sky" centers on Mai Takano, and how she becomes wrapped deeper into Sadako's grasp. "Lemon Heart" tells of one of Sadako's early loves, and eerily wraps itself back into the plotline of Mai Takano. And finally the "Sadako" story goes into the depths of both Sadako's death and her rebirth. Slowly, the details of how The Ring truly works are trickling out into the daylight.