Thief 4 will be an Unreal Engine game featuring a new urban environment and online features, if several Eidos Montreal CVs collated by the armchair detectives at NeoGAF are any indication.
"Now I'm on Thief 4, working with the Unreal Engine," reads Stephane Bonetti's LinkedIn summary. The studio's senior technical level designer Eugene Kuczerepa also lists Epic's engine among his technical credits; he's now "working on various parts of technical level design on Thief 4 aiding and working alongside the level design and scripting teams."
From August 2009 to December 2010, online programmer Colas Fievet helped develop "networking , gameplay, debugging, online and offline tools on Thief 4". From January to September 2011, senior level designer Marc Favreau was responsible for a Thief 4 environment enigmatically known as "The City".
The Thief games have always taken place in urban settings, but have never trifled with online functionality. Before any series disciples kick off, there's no guarantee the "online tools" mentioned are multiplayer modes - this could easily be another Cerberus Network style digital content network.
What appeared to be Thief 4 storyboards surfaced last November.