WARNING: The following contains spoilers for Sailor Moon Eternal: The Movie, now available on Netflix.
Only mentioned briefly in the anime, the Amazoness Quartet plays a major role in the newly released Sailor Moon Eternal: The Movie. While at first they strangely mirror the Sailor Guardians, they are set up as the sub-antagonists that work under Queen Nehelenia. Sailor Moon and her fellow guardians must fight the Quartet back again and again throughout the film's two parts. At the end of Part 2, a major twist reveals the truth about who the Amazoness Quartet actually is.
It turns out that the Amazoness Quartet are actually the Sailor Quartet, originally found lying dormant in the Amazon jungle by Zirconia, who turned them evil. This is a major twist, considering how sacred and important the Sailor Guardians are. It also gives the Dark Moon Circus a true gloom of tragedy, with the group's main soldiers being warriors who were meant to be protectors, not villains, albeit for a future that hasn't quite happened yet.
The Sailor Quartet is made of Sailor Ceres, Sailor Vesta, Sailor Juno and Sailor Pallas, named after four Asteroids in the solar system. They were dormant in the Amazon jungle, sleeping and resting until they were called to action by their one, true leader, Chibiusa. They are her bodyguards in the 30th century when Chibiusa is the main Sailor Guardian. Unfortunately, they are preemptively awoken by Queen Nehelenia of the Dead Moon Circus. Because of this, their evil sides come out through her corruption and their names change to CereCere, PallaPalla, JunJun and VesVes. They then became slaves to the Queen and acted out her nefarious deeds.
When it's revealed at the end of Part 2 that they were the Sailor Quartet, Sailor Moon and her guardians' mission became much easier clearer, with them fighting not just for the planet, but for their fellow guardians as well. Their situation becomes a lot more personal, considering Chibusa's role in the Quartet's future lives. It's also quite tragic for the Quartet -- imagine having to fight against the one person you were sworn to protect. Their internal struggle must have been severe throughout the plot, though not much of it is seen until the end. Their memories were kept hidden and when they started to regain them, Queen Nehelenia sealed their spirits inside four stones.
The Sailor Quartet's role in Sailor Moon Eternal: The Movie is important but also foreshadows future events in this universe. Chibiusa plays a large role in this story and is introduced to Helios, her future love. It makes sense -- if Chibiusa is playing a larger role in the story, then those most closely associated with her would play a larger role as well. While we don't initially know her connection to the Amazoness Quartet, by the end of the movie all of the dots link together.
It's notable that the word "eternal" is used both in the title of the movie and in reference to Usagi's final form, but it can also easily be applied to the way the story incorporates future characters and events, specifically related to Chibiusa. Though their role in this movie is quite tragic, they are rescued by Sailor Moon and the Sailor Guardians and eventually make their way back to the Amazon, to slumber until called upon by Chibiusa.
