From Kotaku:
Eidos-Montreal are in the early development stages for Thief 4, the developer confirmed this morning.
Last week we heard that an announcement was coming, but there’s not much grist in the news that hit today.
Eidos-Montreal, which is currently producing Deus Ex 3, has recruited the "very best core team" for Thief 4, [...]

From gamespot.com:
What we heard: When Eidos Montreal announced that it was working on Deus Ex 3 in November 2007, many fans of the thought-dead sci-fi series rejoiced. Five months later, the same studio sparked rumors that it was working on a new version of Thief, another classic franchise created by now-defunct Looking Glass Studios and [...]