Open LR4, go to File menu > Open Catalog and open your LR3 catalog. It'll ask to upgrade. That bit's wonderfully straightforward!
If you've used the beta version, you can import your beta catalog into your main working catalog by going to File menu > Import from Another Catalog and pointing it to the LR4 beta catalog. At that point, it depends on whether you've used the same photos in both catalogs, and therefore whether you want to keep the LR3 settings, the LR4 settings, or both.