Eric might also use his position of authority. If he's in a position of power within the story's setting, he could simply order Peter to stop. For example, if Peter is violating some rules of the group or faction they belong to, Eric could threaten him with punishment if he doesn't let Tris go.
I'm not sure as I haven't read this specific fanfiction. It could potentially involve Tris being in some sort of danger from Peter, and 'four' (presumably a character) coming to her rescue. Maybe Peter has some malicious intentions towards Tris, like trying to harm her or get her into trouble, and Four steps in to prevent that.
Eric might also see Tris as an asset to his own plans or goals. If he wants to gain more power or influence within their community, he might think that by saving Tris from Peter, he can earn her loyalty and have her on his side. And Tris is known for her strong will and skills, which can be useful to Eric.
In the fanfiction 'four saves tris from peter', the most prominent characters are Four, Tris, and Peter. Four is likely to be a brave and capable character, with qualities that enable him to rescue Tris. Tris is probably a character in need of help, perhaps she's in a weak or dangerous position. Peter, on the other hand, seems to be the source of trouble, the one who poses a threat to Tris, which prompts Four to take action.
In the fanfiction, Tobias might save Tris from Peter by using his physical strength and combat skills. He could have jumped in just as Peter was about to harm Tris, overpowering Peter and getting Tris to safety. Maybe he used his knowledge of the area to find a hiding place for Tris away from Peter's reach.
One common theme is the exploration of their relationship dynamics. Since they have a complex relationship in the original work, fanfictions often dig deeper into how they interact, whether it's as friends, rivals or something more.
Another theme could be the idea of forbidden love. If there are certain circumstances in the original story that make their relationship difficult or against the rules, this can be a popular theme in the fanfiction. Writers can then explore how they deal with this forbidden aspect, like sneaking around or fighting against the system for their love.
Well, I know that some fanfiction platforms have a tagging system. Check those tags for 'tris and eric' and explore the ones that seem interesting. Sometimes, you might find hidden gems that way.
A theme that can't be ignored is the transformation. Sometimes Tris undergoes a transformation, either physically or in terms of her understanding of the world. It could be that she becomes more like a vampire or gains a new perspective on life. And the overarching theme of good vs. evil, with Eric and Tris often on the side of good, fighting against the more malevolent vampires or entities in the fanfiction.