While the Thief trilogy is my favorite game franchise of all time (especially the first one, Thief: The Dark Project,) I would prefer that they not make a fourth one.
Most fans were skeptical of the third one, but thankfully Ion Storm did a bang-up job of keeping true to the originals (even using the same sound effects, the same voice for Garrett, and tying in the original storyline.) Still, though, Deadly Shadows was not quite up to par.
I can't imagine a fourth entry by a completely different staff being anywhere near as good. The best strategy would probably be to completely break away from the old storyline and use a different main character in a different setting.
One of the big appeals was Garrett's 'stealing for stealing's sake' attitude with the plot being a mere afterthought to him (until much later on.) The maps were also very open-ended, promoting exploration of every nook and cranny. If Eidos sticks with those concepts, combined with the quiet, butterflies-in-your-stomach nighttime environments, they might be able to pull it off.