An internet detective at NeoGAF has unveiled some information about Eidos Montreal’s Thief 4 after going over some online resumes of the company’s employees. The forums users claims that the game will, for the first time in the franchise, have some form of an online mode and it will also use the Unreal Engine 3.
The "online" mode was mentioned in this specific Eidos employee's linkedin profile; however, no details about it were given, meaning it could simply be a multiplayer mode or something different entirely.
A few other employees mentioned that they were "working with the Unreal-Engine,” which can be easily deducted that Thief 4 is will be using the popular graphics engine.
Even though Thief 4 was originally announced on May 2009, it's still too early to tell when it will be released or even when we will get any other specific game details.
