Beginning with v3.4, NCover supports .NET 4.0 RTM. Beginning with v3.2, NCover supports .NET 4.0 Beta 1.
Both versions of NCover will correctly cover all of the new features in C# 4 and VB.NET 4. Note, however, that NCover v3.x requires .NET 3.5 to be installed for its own operation. Shipping with NCover v3.2 is a new application in the NCover\Samples folder that you can use along with NCover to see what coverage for the new C# 4 features looks like.
However, even though NCover v3.2 and higher will cover applications that use .NET 4, it does not support the side-by-side feature of the .NET 4 CLR. That is, if an application uses two instances of the CLR, NCover will only cover code that executes in one instance of the CLR, not both. Which CLR instance is covered is determined by the .NET runtime. Support for the side-by-side feature is not planned at this time. You can request the feature if you need it for NCover to work with your program.