Seeing how we have four (4) stories, we basically have four endings for Four.
At the end of "The Transfer," Tobias Eaton has successfully transferred into Dauntless. On top of that, he chooses a new name for himself: Four! This is the start of him forging (should we say four-ging?) his new identity.
When "The Initiate" ends, Four has finished the initiation process and found friends, but we end up with a cliffhanger: he sees a woman on a train that looks just like his dead mother! If you've read Insurgent, you know how this turns out.
Well, you don't have to read Insurgent right away. In "The Son," we discover mommy dearest is still alive. At the end of this story, Four sends her a note saying that he's not going to join her, but maybe he will someday. This is the start of their very complicated new relationship.
Finally, we have "The Traitor," which takes place during the events of Divergent. At the end of this story, Four realizes that he may be a traitor to his family and his faction, but he can do right by at least one person: Tris. It's the perfect ending to make us hungry to read (or re-read) the Divergent trilogy.