Helloes! We are in the finishing stages of Unity 4.0 release.
So here's the third beta build: unity3d.com/unity/beta/download.
A change from previous beta builds: now you can test the free version of 4.0. If you haven't preordered Unity 4 yet, or are using the free version of Unity 3 now -- go ahead, give Unity 4 free a spin from the link above. I guess that makes it the first public beta build.
Enjoy the new features: Mecanim, Flash, Linux, DirectX 11, shadows on mobile, sweet editor improvements, new Image Effects and a metric ton of other improvements. We are pretty proud of how this release is shaping up!
We want some good testing of this build, so please test your project folders and your existing web player games with Unity 4 plugin installed. File bug reports (Help -> Report a problem menu) for anything that is a ship-stopper issue; otherwise we'll only do some finishing touches and Unity 4.0 will be out for real. We aren't calling this build a "release candidate 3" for nothing.
Here are the release notes for Unity 4, including changes from the previous beta builds: unity3d.com/unity/beta/notes (that page doesn't have full notes right now; we're fixing it...)
Stop whatever you are doing and get the Unity 4 public beta now!