For some reason this has been missed so far, probably because of the earlier NeoGaf rumours (http://www.strategyinformer.com/news/16153/rumor-thief-4-to-feature-online-multiplayer). But a journalist actually asked Eidos Montreal about a possible multiplayermode for Thief. Translation from reddit:
http://www.reddit.com/r/Thief/comments/17281f/rumour_thief_4_getting_multiplayer_coop/
original source: http://www.gamer.nl/preview/409245/tomb-raider-multiplayer-wat-hebben-de-deux-ex-makers-er-mee-gedaan
