There is strong speculation on the TTLG forums that it will soon be announced. Edios has used a font almost exactly like the Thief font when talking about the upcoming project, and before announced that a project was in the works side-by-side with DX3, and that its name began with a T. I know, that is grasping for straws, but for a die-hard fan, it is just enough to give hope (or scare the heck out of them "Oh please! Not another Thief 3!" :P).
Hopefully in a few weeks I can turn this into an announcement thread :P, but for now I am interested, who thinks this new project is Thief 4? I admit that it is not a lot to base a belief on, but I think they are just teasing us. :P I got my fingers crossed that it is Thief 4.