The word “Shitennou” translates to “Four Heavenly Kings.” They were likely inspired by the four Buddhist gods collectively known as the Four Heavenly Kings, whose names vary by region and language. Each god represents and watches over a cardinal direction of the world. In Japanese culture, the gods are Tamon-ten (north), Jikoku-ten (east), Zoujou-ten (south), and Koumoku-ten (west). Their respective Sailor Moon counterparts are not officially known, but fans have credible theories. The manga states that one Shitennou governed a specific part of the world: the Middle East (Kunzite), North America (Nephrite), Europe (Zoisite), and the Far East (Jadeite).
The Materials Collection artbook ranges the ages of the Shitennou from sixteen to twenty-six. Zoisite is the youngest at sixteen while Kunzite is the oldest at either twenty-five or twenty-six. Jadeite is eighteen and Nephrite is nineteen.
While it is common for fans to refer to the Shitennou as commanders or generals, they are indeed kings.
They are one of two villainous subgroups in the series to be solely made up of males. The other is the Amazon Trio of the Dead Moon.
The four Shitennou were only seen together once throughout the entire series in episode 44 during a flashback.
In the manga, the Shitennou served as guardians to Prince Endymion in the past. The first anime adaptation excluded this subplot, though. The closest the series ever acknowledged it was in episode 18 during Mamoru and Nephrite’s brief encounter.
Nephrite’s blood was green, but Zoisite and Kunzite bled red. The choice to make Nephrite’s blood that color may have been for censorship reasons as the episode in question (24) was more violent.
For the most part, the Shitennou got along with one another in the manga. Their rivalry was amplified in both the first anime and the live-action series.
In one of Naoko Takeuchi’s illustrations, the Shitennou are romantically paired up with Sailor Senshi: Kunzite/Sailor Venus, Zoisite/Sailor Mercury, Nephrite/Sailor Jupiter, and Jadeite/Sailor Mars. Takeuchi never had the chance to fully explore this idea within the manga. While the first anime and the live-action adaptations both excluded the Sailor Senshi/Shitennou couplings, they were included in Sailor Moon Crystal.