I just saw a post over on Adobe’s Photoshop.com blog where product manager Sharad Mangalick, answered a bunch of Lightroom 4 questions. Everything from catalogs, to video, new features, you name it. Below is one question I pulled directly from the page because I’ve been asked quite a bit too, but there’s lots more in the full article.
@luxx11 and @jennawoodward Ask: @Lightroom why can’t I use my LR3 catalogs in LR4? Is there a way to easily transfer my Lr3 catalogue into Lr4?
Sharad: Since Lightroom 4 is still in beta form, we restricted the ability to bring in catalogs created in earlier versions of Lightroom as a safeguard. This will be lifted when we release the final version of Lightroom 4, and you will be able to import prior version Lightroom catalogs, as well as Lightroom 4 Beta catalogs into Lightroom 4.

It’s definitely worth a read. Here’s the link if you get a chance to stop by today.