Well, I downloaded the trial version of LR4 and I have been working with it for a week, on and off. I have to say that I can see the intended logic of the new PV 2012, but that in practice I am not terribly comfortable with the Basic Panel controls. From the little bit of experimenting I have done, I think that you might be rather more impressed if you were working in colour. However, as I work solely in B/W I am only really concerned with luminance. The thing I really, really miss is the Brightness control. I relied on this mid-tone gamma shift in LR3 enormously, both for global editing and on a grad or brush. Exposure in LR4 is no substitute - I just can't control it so finely, and I can't target it in quite the same way.
However, where LR4 is amazing is in its highlight recovery. With my Hass 3FR files I would reckon it is worth between 1/2 to 1 stop (EV), at least. Which really is food for thought with landscape work, and skies in particular. Now what would have been great would have been LR4 highlights and LR3 controls . . . just kidding, Jeff.