I was recently on a Photo workshop with a well known Lightroom beta tester who writes books on LR. He let slip his new revised book on LR 4 goes to his publisher in Feb 2012 and that we could see a Beta released very soon. He also sugested some big interface changes and a few surprises were to come but was pretty tight-lipped about what He had the beta version of LR4 on his laptop, but of course we didn't get to see it! He indicated that the beta contained only around 60% of the final functionality of the release version.
Judging from comments from Jeff Schewe in the LL "From Camera to Print & screen", soft proofing was dropped from LR3 at the very last minute, therefore I'd expect it to play a significant role in LR4.
Let's hope there's not long to wait.