Several LinkedIn CVs posted on NeoGAF (via CVG) have shown Eidos Montreal is using Unreal Engine 3 for Thief 4, while other profiles suggest the use of online functionality. The term "online tools" is used one of the resumes. The series has remained primarily a single-player franchise up to now, and Eidos Montreal maintaining a singleplay-only stance for Deus Ex: Human Revolution. GAF has the full lot and more.
Despite the acknowledgement of its development, Thief 4 still hasn't been shown. That should change this year, though. Hopefully.