I have no idea when chapter 4 will be ready, my current estimate is 8-12 months, given how busy I'm expecting to get soon.
I do have plans for a v0.3.3 patch which will add a lot of replayability and a bonus sidequest before chapter 4, hopefully that'll be ready within only a month or two.
In the meantime, there is still a good bit of post-gym 6 content available, including a handful of dive spots, Hallio's Crown, the battle facility in Mirable City, and a lot of optional dialogue you can get by going back to old gym leaders. You can also try joining our Discord to discuss how everything fits together with the other members there. There's been some good discussion already, and it's always good to see new takes.