The situation is rather sad.
Sierra owns the rights to the GK characters and name, and they no longer wish to pursue making adventure games. They won't sell the rights to Jane, so she can't make the game with another company
I too used to love Sierra. They were my idol since I was six years old. Now all that is gone, and we're left with a company that makes first person shooters...
Jane would write a GK4 if she could, she said so herself, but she is stuck to writing novels for now.
You should come and join the GK forum at Sierra's website, Doug
It's a great little community there, and you'd be very welcome.
All the best games seem to be disappearing - even MO doesn't seem to be living up to standard