According to LinkedIn resumes, Thief 4's audio director, two lead level designers, a pair of senior concept artists, a senior technical level designer, a senior animator, and a senior level designer have all left Eidos Montreal since 2010.
On top of this, Superannuation writes that Vancouver-based trailer studio Goldtooth Creative worked on a Thief 4 trailer. According to a Goldtooth editor's resume, this trailer was intended for release in December 2011. Footage from that trailer is shown in a demo reel embedded below.
Thief 4 was officially announced in 2009, with Eidos Montreal general manager Stephane D'Astous saying, "We're in the early stages for Thief 4," and noting that the shop was actively recruiting for the project. No official details on the game have come through since.
A Square Enix representative told GameSpot, "We have no comment to make at this time."