John Wick: Chapter 4 feels like the culmination of a story that began in the first movie, so a follow-up would seemingly have to serve as a reinvention of sorts for the franchise.
One theory for the fifth film stemming from Stahelski's own comments, for example, posits that John Wick, the assassin, has truly died, but John, the man, lives on.
This would mean a story in which John has left his assassin life behind yet again, potentially opening up the door for John Wick 5 to tell a smaller and more intimate story with a somewhat different tone.