NeoGAF users suggest the game will be running on Unreal Engine 3, and will feature an online multiplayer component.
Several of the profiles state that Thief 4 runs on Unreal Engine 3, which isn't too surprising given Square Enix's recent UE3 licensing deal. Among the profiles is an online programmer who is "involved in many aspects, such as networking, gameplay, debugging online and offline tools on Thief 4." The programmer in question specializes in game networking (matchmaking, etc.), so online play may be a consideration.
We haven't heard an official peep out of Thief 4 since November of 2009, so who knows what form the game has taken by now?