More information on their site http://unity3d.com/
The announcement on their blog :
Im really happy to tell you all that Unity 4 is officially released.
We have been hard at work putting the final touches on this first release in the Unity 4 cycle, and were extremely happy with the results.
Before going into the details (but perhaps after you start your download), let me try to define what we think Unity is and what it should be.
Unity is a dream weve had for years. A dream of packaging up the fanciest game engine technology into a bundle thats extremely approachable, affordable, and still flexible and powerful enough for the most demanding users. And a dream of running an important and long-lived company that helps game developers all over the world as much as we possibly can.
A big dream, sure. But the proof is in the pudding and we have come a long way: from three guys in the basement 10 years ago to a company of over 200 brilliant yet humble ninjas all over the globe, who spend most of our waking time serving the game developer community.
Steve Jobs once said that victory in our industry is spelled s-u-r-v-i-v-a-l, and we are still here 10 years later, doing the same as we have always done
just more of it, and for more people!
Thank you so much for being with us thus far.
Unity 4.0 marks the release of the Mecanim animation system and we think its mind-blowingly awesome. Its going to change the way a lot of people make games by giving high class animation tools to a much wider range of developers than ever before. Were also putting up free motion captured animation on the Asset Store so that you can start making your characters move with lifelike fluidity.
The wealth of new high-end visual features will level the playing field. DirectX 11 support is probably the biggest addition that will open up a huge range of possibilities for developers. Add to that mobile real-time shadows, normal maps when baking lightmaps, skinned mesh instancing and theres a wealth of lush visual features.
Finally (for the really big stuff anyway), two new supported platforms have made their way to Unity.
Linux Deployment Preview
The Linux developer preview is already looking amazing and is helping games make their way onto one of the most open gaming platforms in the world. Rochard was recently released on Linux and demonstrates that Linux development can be very successful.
The Adobe Flash Player add-on is going to push Unity way ahead of the rest of the industry in potential publishing reach. The combination of Unitys ridiculously broad platform support with the Flash plugin being installed on over a billion computers means that anyone who uses Unity can reach an incredible array of potential customers.
Unity 4.0 is a great beginning, but we are here for the long haul and were going to make Unity better and better throughout this release cycle. As we said in the original announcement, our goal is to have releases come out at a faster pace because we think it serves you better to give you access to awesome new toys (and fixes) faster than ever before.
You already know some of what were working on. Things like the new retained GUI, Windows Phone 8 support, and Nintendo Wii U support. Down the line you can expect to see some pretty big additions that we cant talk about quite yet, but that we know youre going to be very excited about!
2012 has been a big, big year for Unity and were looking to make 2013 even bigger. Enjoy Unity 4.