When we first rejoin Eleven in season four, she's trying (and often failing) to settle into her new life with the Byers in California. After being mocked one time too many, she injures one of her bullies and gets arrested, only for her "Papa," a still-alive Dr. Brenner, to show up and tell her that her fate isn't over yet.
As she works with Brenner to regain her powers, we learn more about her past in the Hawkins lab, eventually learning that she's the one who fought off "One," a brutal fellow test subject, and cast him into an alternate dimension where he became Vecna and created the Upside Down.
The end of the season sees Eleven and her friends teaming up to take down Vecna, and while they succeed in stopping his immediate rampage, they fail to kill him or stop him from opening a gate from the Upside Down into Hawkins, leaving Eleven with at least one more epic confrontation to come in the future.