Thief 4 seems to be utilizing some form of online features, if the CVs of some Eidos Montreal employees are any indication. If these early hints are true, Thief 4 will be the first game in the series to go online. Details are scant, and online features haven't been officially announced. However, given the proliferation of online passes and DLC, it seems like a reasonable expectation these days.
Users of the forum NeoGAF (via CVG) spotted an employee from Eidos Montreal listed as an Online Programmer. The profile specifically mentions "networking" and "online tools" for Thief 4. That doesn't necessarily mean an online mode will be offered in the new Thief, of course.
While the evidence suggests Thief 4 will have an online mode, Shacknews has contacted Square Enix to confirm the rumor. We'll update as more information becomes available.