I for one, find the idea of a 4th game being a prequel to be a bad idea. 3 opened up new possibilities for stories and characters. New monsters arriving via the second conjunction, a possible Northern Empire which may or may not last if the conquored Kingdoms rebel, or Nlifgaard ruling everything, which also may or may not last if there's rebellion. I suspect it may have to do with Geralt likely being retired, or different endings to choose from. Nothing a game import can't solve. Plus, Doug Cocker himself has said that he thinks Ciri should be the next protagonist.
But I'm open. For those who wish to have a prequel game, let it out. Unlike the world of the Witcher, we can try and get along.