We know for a fact that it's in development; Eidos has said as much. Now the question is, what is the plot going to revolve around.
Any chance we'll be playing that girl "Keeper" Garrett caught trying to rob him? My guess is that he saw potential in the girl that he himself once had, since she was able to see him and almost got away with his money (and Garrett himself did say "it's not easy to see a Keeper, especially one who does not wish to be seen."), and so takes her under his wing and teaches her how to be a skilled thief. So I would say it's definitely possible that girl will be the main character in Thief 4.