There is a pretty comprehensive review of Lightroom 4 Public Beta over at DP Review:
The Lightroom 4 beta introduces quite a list of features, including a completely new book-creation module, expanded support for video, soft proofing capability, and geo-tagging of still and video images via a Google Maps-powered module. Image editing tools have also been significantly updated, with a new process version (PV2012) that includes a reworking of the Basic panel controls and new localized editing options.
via Lightroom 4 Public Beta: What’s New: Digital Photography Review.
There are some tasty features that I’m itching to try, but I’m a little wary. I’m gonna wait until somebody else using Lightroom 3 installs this beta and gives it a going over. I’d hate to lose work because of a janky beta.