When I was first introduced to (the Japanese) Sailor Moon, I saw "Shitennou" as nothing but a name. As I learned more, I began to find it odd that the commanders of the Dark Kingdom would be called the "four heavenly kings".
Admittedly, it makes more sense in the manga, where they start out as Endymion's guardians, but still...
However, as I saw the Fushigi Yuugi OAV where characters carrying the same title appear, I understood that there most be actual Shitennou somehwere in the japanese mythology.
So, who are the kings of heaven?
The Shitennou belong to a group of Buddhist deities which were imported from other religions, in this case old Indian mythology. They appear mostly in the earlier hindu texts, specifically the twenty-sixth chapter of the Lotus Sutra, one of the Veda books.
The shitennou came to Japan with the first wave of Buddhist missionaries, but quickly became quite popular, and statues of them have been dated back to the seventh century.
Acting as protectors of religion, guardians of the four directions and generals under the deity Taishaku (Indra in sanskrit), it is easy to see why the Shitennou are pictured as warriors, wearing full armor and holding weapons.
When upholding Buddhist law and protecting those who follow the sutras they are invincible. To show this, they were often portrayed standing on fallen monsters.
Statues of the Shitennou are very popular in shrines and temples, and are usually placed in each corner of the temple or around especially holy places. One prays to them for mercy, and in older days warriors could also ask them for luck.
The Shitennou are said to live on the mountain Shumi-sen which is a mythological place. According to Indian tradition it is located in the middle of the world.
The peak is the home of Taishaku and halfway up, in their respective direction, each Shitennou has a kingdom. Together their kingdoms make up one of the six heavens of the world of desire.
Jadeite represents Jikoku or Jikokuten, the guardian of East, whom functions as the protector of the world. The other three each have their own main duties.
Since blue is the color of East according to Buddhist tradition, this is the color Jikoku is normally associated with, and his statues are always placed in the eastern corner of a shrine. It is interesting to note that Jadeite, in the manga, is shown with a blue-striped uniform, opposed to the anime where his color is red.
In sanskrit, Jikoku's name is Dhritarashtra, and no, I don't know how to pronounce it.
Although the concept of Shitennou is something purely Buddhistic, Dhritarashtra appears already in the Hindu epic Mahabharata. This is one of the most holy texts in the Hindu culture, and describes the wars between the Kauravas and the Pandavas. It also shows how to behave morally correct, and asks as well as attempts to answer several of the large questions in life.
However, Dhritarashtra is far from the Jadeite we know and love... the tale portrays him as an blind, aging king. Not to mention father of a hundred sons! But, true to his nature *wink* he's on the side of the 'bad guys'. In the end, he goes with his wife into the forest, where he dies during a fire.
Interesting as mythology is, I think I prefer Takeuchi's version ^_^
Spelling: Shitenoh, Shiten'ou, Shiten, Shi Tenno. Shi means 4 and tenn is a word from Chinese; it's literal meaning is heaven and it is often used to indicate holiness. Ou means king (as in Haruka Ten'oh = distant sky king)
In Japan, one uses -ten as a suffix to indicate holy origins and respect.
Four aknowledge masters of different crafts or arts (such as tea-ceremony for instance) might also be honored with the title Shitennou sometimes.
For those interested in the Shitennou, I can recommend following animes: RG Veda (by CLAMP), Fushigi Yuugi OAV by Yuu Watase and Legend of the Four Kings. Note that I haven't seen the last yet, so I don't really know what it's about ^.^;;
Unfortunately, Jikoku-ten never shows up in RG Veda, having been killed before the story takes place (damn)
I'm in no way an expert on Buddhism, so if you find any errors, or have a question, please mail me. I'll help out the best I can.
Disclaimer: Sailor Moon © of Naoko Takeuchi.