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I recently learned that you can type "GK4" into Sydney and it well tell you about ghosts. So apparently there was supposed to be a "Gabriel Knight 4" about ghosts. I guess it wasn't made because Sierra went downhill. I wonder if it ever will be made.
I could be mistaken but I thought that Jane Jensen + some others had a GK4 page up for quite some time and were working on it conceptually but there was never enough interest from outside parties (fans included) and so they recently dropped / gave up on the project.
It's been speculate that if the GK1 remake would have done well sell wise it will lead to a new game in the series, however as Jensen closed her studio, Pinkerton Road, and effectively retire from making game it will never happen.
That sucks. Not terribly surprising considering we're talking about a 20+-year-old series of games that wasn't enjoying "King's Quest" popularity even when it began, but that still sucks.

Maybe there will be a fan-made GK game or a fanmade game loosely based on the concept? There are fan-made King's Quest, Space Quest, and Quest For Glory games so it can happen.