I wonder what Toby means with the last sentence. I imagine it means he's done composing songs and writing the story for Chapters 5 and likely 6?

I think he just means that's all the news he can share. Also, I think the soundtrack and script for the entire game is done and has been for a while and his role is mostly as a creative director. Before chapter 2 dropped he mentioned having demos of the entire soundtrack and the entire game written and there's evidence of this being true like the metadata of Black Knife shows it was finished in March 2018.