Here's the status of every known test subject in the Stranger Things universe ahead of season 4. The character of Eleven has been the most significant test subject involved in the Netflix series, having been experimented on at Hawkins Lab before making her escape for a chance at a normal life. Through Eleven's backstory and the introduction of tie-in graphic novels, several other test subjects have been added to the Stranger Things Extended Universe.
Prior to the events of Stranger Things, Dr. Martin Brenner enlisted a number of children with special abilities to be the subject of Project Indigo. The subjects resided in Hawkins Labs as their powers were tested through experimentation. As many of the participants got older, they realized that Dr. Brenner was involved with something much more sinister. Dr. Brenner was forced into hiding and the lab was eventually shut down, but several of the test subjects are still in existence in the current timeline.
One of the lingering questions surrounding Stanger Things is the mystery regarding the very first test subject's identity. Considering Eleven's identifying number, there had been ten other test subjects before her, but only a handful have been introduced. Based on what's known about their current whereabouts, there's reason to believe the characters will appear in the Netflix series in the future, and more test subjects could possibly join them.
Three (Ricky)
Ricky, also known as Three, was not only the oldest test subject as part of Project Indigo, but he's also the only male out of all the known participants. He was a prominent member of the prequel comic, Stranger Things: SIX. He met a girl named Francine in 1976 and realized she too has special powers. After starting a romantic relationship with her, Ricky convinced Francine to allow Dr. Brenner to help her at Hawkins Lab. After agreeing to the experiments, the duo started to have opposing viewpoints on Dr. Brenner's special project. Due to Ricky's unwavering support of Dr. Brenner, there was speculation that the two are related.
At the end of SIX, Francine led an escape attempt from the lab. She ended up sacrificing herself to save Ricky and another test subject. Ricky was last seen fleeing into the forest with another test subject after the big escape. As revealed in the latest comic series, Into the Fire, Ricky survived the escape and has recently reunited with other test subjects for a special mission.
Six (Francine)
Francine was the sixth test subject within Hawkins Lab's Project Indigo initiative. The character, who was at the center of Stranger Things: SIX, possessed the power of precognition. In 1976, her family moved next door to Ricky and quickly learned that he had the ability to manipulate emotions. After becoming his girlfriend, she agreed to join Dr. Brenner's program to figure out the full potential of her powers. Francine began to get visions of large monsters and realized that danger was on the horizon. She found that the experimentation was much more dangerous than first thought, so she came up with a plan of escape.
After she envisioned Ricky getting shot by a guard, Francine sacrificed herself so that Ricky and another subject can safely get out of the lab. Dr. Brenner told her that his team will find the others but she claimed to know that his fate is doomed now that she changed the future. Six ended up succumbing to her wounds, so there's no chance that the character could end up in Stranger Things season 4.
Eight (Kali)
Besides Eleven, Kali is the only other test subject to appear on the live-action series. The character, also known as Eight, was already a member of Project Indigo when Eleven was born. The two became extremely close, considering each other sisters. When Kali's strength of illusion casting and clairvoyance became strong enough, she escaped from Hawkins Lab. She went on to find a rag-tag group of followers as seen in the Stranger Things episode titled "The Lost Sister." She eventually reunited with Eleven and tried to convince her "sister" in her journey of revenge, but Eleven left her behind when she realized that her loved ones from Hawkins needed her.
According to Into the Fire, Kali and her crew have taken refuge in a dilapidated barn in Hastings Glen. The 20-something is still looking to get back at the people who hurt her and the fellow test subjects. Kali is met by Ricky and another test subject and reveals to them that a "sibling" previously presumed dead is actually alive. The girl is being held in a hospital or facility, and it's very probable that Dr. Brenner is involved.
Nine (Jamie)
In addition to the other test subjects mentioned, Nine and her twin sister served as characters in Stranger Things: SIX. The character named Jamie came to the lab with her sibling after the rest of their family died in a fire. Jamie had the power of heat generation and starts to learn the potential of her powers. At one point, the others watched as Jamie perform a test. She ended up losing control and was sent to the facility's hospital where she remained in a coma. It's insinuated that she had no chance of waking up, which was why the rest of the group left her behind when they tried to escape.
Jamie was thought to have died following the events of SIX, but the latest Into the Fire series revealed that the test subject is actually alive. Kali learns of her survival and shares the intel with Ricky and Jamie's twin sister. Nine is supposedly being held in a facility and on heavy medication, suppressing her pyrokinetic abilities. Her "siblings" decide to team up to rescue her but matters become more complicated when she burns down the mental hospital that she was being held in. Jamie goes on the run but her story will continue to play out in the future final issue of Into the Fire.
Nine-Point-Five (Marcy)
Jamie's twin sister, Marcy, was identified as Nine-Point-Five in Project Indigo but she never exhibited any powers during her time in Stranger Things: SIX. Like Jamie, Marcy became good friends with Francine and an acquaintance of Ricky. When Jamie went into a coma, Marcy didn't have much hope that her sister would wake up. She agreed to escape from Hawkins Lab as orchestrated by Francine. Jamie and Ricky managed to flee from the lab when Francine sacrificed themselves.
Nearly seven years later, in 1985, Marcy is still with Ricky as the two are rounding up the surviving test subjects now that the lab is closed. Their search in Into the Fire takes them to Hastings Glen where they find Kali. Marcy learns that Jamie has been alive all this time so she quickly gets on board with the plan to get back to her twin sister. Unfortunately, Jamie burns down the hospital before her sister can reach her but she continues to track her down.
Eleven (Jane)
Eleven continues to serve as one of Stranger Things' central characters. As the seasons go on, more of her backstory is revealed including how she was born into Project Indigo after her mother, Terry Ives, participated in Project MKUltra. So far, the girl with telekinetic powers has only reunited with one other test subject since getting away from Dr. Brenner's control. Since she was so young during the experimentation, it's possible that she doesn't remember the other participants but they will probably remember her.
It's very likely that Ricky and Marcy's quest in bringing the test subjects together will lead them into Stranger Things season 4 or beyond. Eleven, however, is no longer living in Hawkins since she moved with the Byers family to an unknown location. It's possible that her new home will be closer to her "siblings" or they will all go back to where it all started.