The Nina Project seems to have done its job in making sure Eleven is powerful enough to at least stand a chance of squaring up against Henry Creel's own abilities, as she's once again able to enter The Void, has regained her telekinetic abilities, and seemingly. Vecna relies on the negative memories of those he marks for death in order to kill them, something seen in the fact that he uses Max's memories of Billy against her, and thus it makes sense that the series of positive memories of Max's life.
It's important to remember that the scenes of Eleven using her powers to bring Max back to life don't happen in the real world, which suggests that she hasn't been able to revive the dying just in time for the final season. As Lucas desperately tries to convince Max to live in the real world, Eleven's connection with Max via The Void allows her to also take in her friend's dying moments, before refusing to let Max die and recalling a series of memories the pair had together while trying to use her powers This type of motivation has been seen before in the show, when Eleven used the memories of her mother in order to send Henry Creel to the Upside Down in the first place - and also seems to be how Mike's confession of his love assists her, as well.
After leaving audiences to stew on the events of vol 1, the final two episodes of Stranger Things season 4 proved even more dramatic than many viewers were expecting, tying the plots of the various groups together and bringing them back to Hawkins to set up for the thrilling conclusion of the beloved Netflix TV show. While the team are able to hunt him down because of her sacrifice, Max begins to succumb to the villain, showing signs of becoming this fourth victim and the final death needed to doom Hawkins to the Upside Down's forces.
If she is able to use her connections with people and the love she has for them in relation with her powers, she could potentially become more powerful in the long run.
It's worth noting that these memories are all of Eleven and Max's time together, instead of the memories she is able to access from Max's mind, suggesting that it may be Eleven's own emotional reaction to these events that allows her to resurrect Max. It makes sense that Eleven would be able to battle Vecna's abilities in order to bring her back and prevent a permanent Max death.
The finale confirms that Max died in this scene and that Eleven used her powers to revive her. It makes sense that the inverse could also be true, and that recalling positive memories and emotions allow Eleven's extraordinary powers to aid a friend in need. Though Max ends the season in a coma, she is brought back to the land of living seemingly by Eleven, which raises a lot of questions as to how this new power works. After her training, Eleven is possibly even more powerful than she's ever been before, which she'll definitely need, given what's been set up for the events of the final season. Even though Eleven's powers come back, she also seems to use them in a completely different way in the end of the season.
This is because Eleven uses her powers to seemingly resurrect Max after her decision to use herself as bait for Vecna goes awry. With the show's final cut to two days after showing Max alive, it would seem that Eleven intended to use her powers in order to revive Max. The fact that Eleven survived his own murder supports the idea that he is less powerful than Vecna. She's come a long way since the beginning of the fourth season of Stranger Things, when she was trying to regain her powers. At the end of the season, Eleven seems to use a new ability, which is notable since all the cast evolve throughout their respective arcs.
Throughout the course of the show, Eleven's abilities have been made clearer, but season 4 focuses the most on them. With season 3 ending with Eleven seemingly losing her powers, much of her arc this season has centered around her simply regaining any powers, with actively training them being comparatively a wayside goal.
Major plot twists for the series include a new power for Eleven. This would explain how Eleven was able to beat Vecna in Stranger Things season 5 because he cut himself off from human connection. The answer is related to how Vecna's powers work with Eleven's.