All of Chapter 4 is new content, and thus are not part of the Quest lists. Chapter 4 is also the latest issue for this game, so new content will be announced when the Devs decide to drop another update. The dragon quest therefore is the last major quest that is available to players. (After the decay quest is the dragon quest).
Technically, there won't be a 4th person, though I would love for that to happen. You'll see other followers pop in and out, but they are temporary compared to your main three, thus they won't have the special ability that your main three will have.