Yea, thief 3 wasn't a huge disappointment, it just deviated from the previous games a bit, and thief, with the huge cult following it has, is bound to raise more then a few eyebrows if it deviates even a slight bit from the beloved and colossal shadow thief 1 & 2 cast.
I'm rather optimistic about it actualy, sure it will deviate from the original formula a bit, but you can't bring innovation without changing or adding a few new things to the original. Dishonoured gave new life to modern stealth based games. You could play it without powers and it had a definite thief feel to it, altough the setting wasn't as interesting and the levels not as big or complex.
If they're going to give thief 4 an open city, I hope they dont go the assassin's creed way of arbitrary alternative gameplay elements that, while somewhat fun as a distraction, have no point gameplay wise. In assassin's creed you could gather money to buy things to make more money to buy more things, an essentialy pointless effort since you could get everything you'd ever need without ever bothering with that feature. Also I hope that they preserve the massive indoor level design with complex levels and multiple routes, forcing you to organically adapt to every level. Thief is the kind of game that should tell you to rob a huge mansion, while only giving you a hand drawn map and some minor tips, and let you sort yourself out, not putting some silly waypoint and telling you how to overcome every obstacle.