OK, first of all I don't think that we can compare Thief and Dishonored very well. In terms of level design - OK! In terms stealth - OK! But everything else is kind of a different thing in it's own right.
I personally think that Thief 4 is going to be a very good game. Not a great one, definitely not better than The Metal Age but a pretty damn good one. I have faith in the developers after their recent success but I don't think it is going to be anything groundbreaking given their abilities and the time they are given.
I want to personally say that no matter what Thief 4 is going to be (even if it is great and groundbreaking) it's still not going to be better than Dishonored TO ME PERSONALLY. Only because of one thing. Aesthetics and graphic design. Hardly anything will actually, unless they make a new Deus Ex that has magic...joking, of course. To me it's just that games nowadays just bump up the polygon count and the textures and there is no identity behind that artwork. That's the biggest reason I fell in love with Dishonored.
I hope that both games are a great fun to play and I hope that fans of both titles are not disappointed. Cheers!