Dark Kingdom
Before the events of the series, the Four Kings of Heaven originally functioned as the four guardians of Earth's crown prince Endymion. They were led astray during the time of the Silver Millennium, selling their souls to the Dark Kingdom and becoming Beryl's servants. When reincarnated in the 20th century, they again fall prey to her and obey her every command, but retain a sense that they need to seek out their real master and aid him. The Sailor Guardians fight and kill the four of them, one at a time. However, they are all briefly revived near the end of the "Dark Kingdom" arc and are able to remember the Golden Kingdom and their time as Prince Endymion's guardians before being turned into the stone from which their names are derived. The stones are kept by Mamoru Chiba (Endymion's reincarnation in the present day), who uses them to call upon the spirits of the Four Kings of Heaven for advice.