In a recent blogpost by Eidos Montreal it was revealed by the studio's Adam Badke that Thief 4 will remain true to the franchise's roots, with the game's main protagonist - Garrett - able to progress through the campaign without having to kill anybody, should the player so wish.
In the post Nicolas Cantin is also quoted as saying: "It's not correct to say that we're trying to make Thief ‘mainstream', or that we're trying to make Garrett ‘less gothic'... This isn't the case."
"I was referring specifically to a previous Garrett design we tried out internally and not Garrett from the previous games. Our early design went a LOT more gothic - with black nails etc - but we thought that this wasn't true to the legacy of Garrett so we pulled it back a bit. Returning to something more true to the original Garrett is what I meant when I said we made him more ‘mainstream', this wasn't a comment about the direction of the game."
That should put to bed community fears that Eidos Montreal were taking the game too far away from its original premise.