At Build MVC4 was announced and shortly thereafter it became available for everyone to play with.
How Do I Get It?
You can get it three ways:
What’s In It?
MVC4 comes with many great new features including an updated face lift of the design:

Mobile rendering baked in:

You can see the side by side comparison of the old MVC default template and the new. The newer template is just cleaner and easier on the eyes.
If you need more mobile then use the Mobile Project Template that uses jQuery Mobile!
Superficial?
These changes are just superficial. True, but they do have more features such as recipes and task support for asynchronous controllers. You can also install MVC4 side by side with MVC3 without issue.
Upgrading MVC3 to MVC4
No Problem. At least in my case. I did it both ways as described in the ASP.Net MVC 4 release notes.
The recommended approach is to create a new MVC4 project and move your controllers, views, etc… into the new project which works but if you have a large project that would be a pain.
The other option is to do it all manually (as described in the release notes) or use nuget to install the MVC4 bits and make a few less manual changes.
After installing the MVC4 package in your application, you will want to make all of the manual edits as outlined in Upgrading an ASP.NET MVC 3 Project to ASP.NET MVC4 section of the release notes with the folowing exceptions:
- You don’t need to do step 3. The Nuget package takes care of this.
- On step 7, I had to add a binding for Razor
<dependentAssembly>
<assemblyIdentity name="System.Web.WebPages.Razor"
publicKeyToken="31bf3856ad364e35" />
<bindingRedirect oldVersion="1.0.0.0" newVersion="2.0.0.0"/>
</dependentAssembly>
Should I Care?
Just the inclusion of mobile baked in is a big win and should be a good reason to watch MVC4 take shape over the next several months.
I know this is early and I’m sure more features will make it’s way into MVC4. At the pace that MVC has been maturing I am excited to see what’s next!
Happy Coding!