The team behind Stranger Things is keeping details of season 4 a closely-guarded secret, but recent reports about the series adding four more characters suggest the new season could introduce more test subjects. The Duffer Brothers’ hit sci-fi series Stranger Things has been building its own mythology since season 1 debuted on Netflix in 2016, but it has also been leaving a lot of questions and mysteries that fans hope will be addressed (and solved) in season 4.

Season 3 was a big shake-up for the characters in the series as Eleven lost her powers, Billy sacrificed himself, the Byers (and Eleven) moved out of Hawkins, and Hopper died (but not really). Stranger Things made sure to leave one final tease at the end of season 3 by showing a military base in Kamchatka where the Russians have an “American prisoner.” This has paved the way for all types of theories regarding Hopper’s return, who the American prisoner is, and the possibility of characters from past seasons (namely Dr. Brenner) coming back.

Related: Stranger Things Theory: Eleven Lost Her Powers Because Hopper Had To Die

Among the many things fans would like to see in season 4 is more test subjects from Hawkins Lab. Stranger Things season 2 revealed that Eleven definitely isn’t the only one, and there are at least 10 more out there, somewhere. In that same season, Kali (or 008) was introduced, met Eleven, and then was completely ignored in season 3. But a recent report on new additions to the cast for season 4 could hint at more test subjects making an appearance.

Stranger Things 4 Will Add Four New Characters

Brenner watching an experiment on Stranger Things

Stranger Things will reportedly add four new recurring characters. All of them male and three are teenagers. The adult character has characteristics that suggest he could be one of the Russians, and the teenagers are described as a metalhead, an “entitled jock” (1980s stereotype, anyone?), and a stoner. Taking into account that Kali has been living her life outside Hawkins Lab for years, it’s possible that the aforementioned trio of teenagers are other test subjects that also made their way out of the lab. So far, the only subjects Stranger Things has featured have been female (Eleven, her mother Terry, and Kali), and it’s hard to believe that the Lab only experimented on women.

Of course, these characters could also be Russian test subjects, which makes sense given that season 3 established that there are some dangerous experiments going on on the other side of the world. It wouldn’t be surprising if the Russians have been experimenting on kids and teenagers just like Brenner and company did in Hawkins – and if Brenner truly has been living in Russia all this time, then it would make even more sense, as he could be the one behind these experiments (again). Stranger Things season 4 is still in pre-production, so the descriptions of these characters could change without notice, but hopefully, the upcoming season won’t keep ignoring the fact that there are other test subjects out there that viewers haven’t met and that could be of help to the rest of the characters.

Next: Stranger Things: Every Kid Experimented on by Hawkins Labs