Thief 4 is currently in early development, and right now there's almost no information about what the game is actually going to be like. I hope we'll be getting some more specific details soon.
Has anyone heard of the Master-Thief Lytha recently? She has an entire way of playing Thief. A way where you must find all Secrets, steal all Loot and other items and cause as little damage as possible, all on Expert difficulty.