One particular LinkedIn profile belonging to Colas Fievet, for instance, listed among his duties for the project "networking, gameplay, debugging, online and offline tools on Thief4."
Of course, online multiplayer is only one possibility for that kind of functionality. Other features that might require online networking and gameplay include tools for auto-installing downloadable content and mods, as well as expansions. Other services might require live streaming internet.
The other thing gleaned from the LinkedIn profiles of those who worked on Thief 4 mention use of the Unreal Engine 3. While some amazing looking games have been created using the engine, like Batman: Arkham City as well as the upcoming Mass Effect 3, the Thief games heavily relied on specialized engines that measured light levels, like the first two games' Dark Engine.
While some may fear that the publisher and developer are bowing to pressure to include multiplayer like so many other games - BioShock 2 is one oft-cited example - it must be noted that Deus Ex: Human Revolution lacked multiplayer.
Square-Enix has yet to confirm or deny online multiplayer for Thief 4. Stay tuned.