Great !
I'm glad the game is still actively updated, but don't delay the release indefinitely ;). Maybe an early release on Steam and other would boost the game and the team.
Regarding Godot itself, the more you wait, the more it will be painful to make the leap as 4 branch diverge more and more from the 3. The only advice I can give you is to upgrade the project scene by scene and script by script, it would be more sustainable in the long run than trying a full upgrade at once which is doomed to fail, 100% garantee. Many objects have new/less properties, have been renamed, there are tons of breaking changes made, even small demo project are pretty annoying to upgrade. Let's just hope the design allows such procedure, if all scenes are trully independant from each other.
Still, going over the difficult time will greatly help the game: better and more compatibility and better performances. It will also show to the users that the team is commited. Nothing really could justify to stay with the 3 branch unless maybe an immediate release NOW.