The Shitennou are the first enemies that Sailormoon ever encounters. However, they weren't always the enemy. These four have a very strong tie with Prince Endymion, which is not something that is really covered in the anime. The manga, on the other hand, delves much deeper into the topic. So just who are the Shitennou? Just as Princess Serenity had the four sailor guardians, Endymion's own guardians are the Shitennou.
It is said that they were very close and the generals were very loyal to their prince. When the Dark Kingdom began its attack on the Moon Kingdom and all were killed, the generals were reborn and brainwashed by Beryl, becoming her servants. However, after Usagi begins to regain her memories as Princess Serenity, one of the generals - Kunzite - remembers that he used to serve Endymion, not Beryl. Alas, it is too late for them, the other generals have already been destroyed, and Kunzite's state of mind does not last long because Queen Beryl brainwashes him again.
When Mamoru is stolen by the Dark Kingdom and turned evil, Sailormoon has no choice but to fight him - and when she realizes that part of the ginzuishou is inside of him so she cannot fully heal him, she must kill him. She then turns the sword on herself.
However, this is not the end for the lovers. Mamoru had the stones which contained the souls of the four perished shitennou in his breast pocket. They blocked the sword and saved their prince, even in death. The ginzushou restored Sailormoon. Sailormoon tried to fight against Metallia, but was having trouble defeating her. Feeling useless by her side, Mamoru was unsure of how to help. Then the spirits of the four shitennou appeared to Mamoru. Kunzite said to focus on Metallia's weakspot, her forehead. Even in death and after various brainwashings by the Dark Kingdom, the shitennou were still loyal to their Endymion-sama. Mamoru keeps the stones with him his entire life, carrying the souls of his servants and friends by his side.
The Man: Jadeite was the first of the Shitennou to appear. As such, we are not shown a great deal of his personality. Judging from his actions in the manga, he seems very respectful and loyal to Queen Beryl. He seems very serious about his work. I believe it is safe to assume that his personality was probably the same when he served Prince Endymion in the past.
The Stone: Jadeite Jade is considered the more precious or "true" form of jade, as opposed to nephrite jade. Jadeite jade, a pyroxene, is mostly from Central America, as opposed to nephrite jade which comes from places such as China. It comes in vibrant green colors and has a fine translucency to it. It is often a preferred material for carving.
The Man: Nephrite is the second shitennou to appear. He's not around in the manga for very long, but we see him a few times with a crystal ball. He seems to have the ability to possess others, such as Princess D. In the anime, he is a much deeper character. Towards the end of his run, he begins to fall in love with Usagi's friend Naru, but is killed by Zoisite before Neprhite can really turn away from the Dark Kingdom. In the manga, however, Zoisite does not plot Nephrite's death, and is actually visibly upset when he is killed by the sailor senshi.
The Stone: Nephrite is the less valuable form of jade. While jadeite jade is known for its vibrancy, nephrite jade usually is identified by less intense colors such as dark green, brown, or black. It is usually mined in places like North America, Australia, and Taiwan.
The Man: Yes, first of all I'd like to make certain that you know that Zoisite is indeed a man. In the North American dub of the anime, Zoisite was changed to a woman because of his intimate relationship with Kunzite (called Malachite in the dub). Zoisite does give off a slightly feminine vibe in the anime, however. He is associated with the sakura petal in the anime. In the manga, however, he does not have a romantic relationship with Kunzite at all. He also does not have the same animosity towards Nephrite or Jadeite like he does in the anime.
The Stone: In color, the stone has some variations, but the most well-known color associated with zoisite is green. It usually has transulcency or even transparency to it. It has been mined in locations such as Tanzania, Austria, and Switzerland.
The Man: Kunzite is the last shitennou to be seen, thus the last to die. His story deviates a bit from the others, however. In the North American dub his name is changed to Malachite. In the anime, he is in a romantic relationship with Zoisite, which does not occur in the manga. Towards the end of his time in the manga, Kunzite's memories of his past life serving Endymion begin to resurface. Before he can really act upon this, he is brainwashed again. He is the only one of the shitennou to recover some part of their memories before they die.
The Stone: Kunzite is a pale pink gem named after George Kunz, a gem scholar who lived at the turn of the 20th century. It was first discovered in Connecticut, USA, but today is mined in places like Brazil and Afghanistan. Malachite has very distinctive light and dark green bands on it. It is very opaque and has a dull luster to it.
Peppermint Storm - The best site around for tons of in-depth information on the Shitennou, delving much deeper than this little old page.
PGR-Gems & Minerals Data Sheet - I found information on the Jadeite, Nephrite and Kunzite stones here. Has information on tons of other minerals and gems.
The Mineral Zoisite - I found my information for the stone Zoisite from here.
The Mineral Malachite - I found my information on the stone Malachite from here.