It is common to shorten 4.0 to 4.
This is a response to Adrian Grigores comment "The .NET version does determine the syntax and semantics of the C# language".
There are (at least) three partially independent version numbers - the .NET Framework version, the Common Language Runtime (CLR) version, and the C# version (see this StackOverflow question for a quite comprehensive list of Framework and CLR versions).
The C# version determines what language features are available. Language features are based on .NET Framework features - the included framework assemblies and CLR version. Finally every .NET Framework versions includes a specific CLR version that basically determines what is valid Common Intermediate Language (CIL) code and how it must be interpreted. Some examples.
C# 3.0 introduces automatic properties. This feature is build into the compiler and does not rely on new functionalities in the .NET Framework assemblies or even the Common Language Runtime (the .NET Framework 3.0 still contains the CLR 2.0). Therefore it is possible to build an application using automatic properties and targeting the .NET Framework 2.0 (maybe even 1.0 and 1.1).
C# 3.0 introduces LINQ. This feature is mainly build into the compiler but partly relies on assemblies new in the .NET Framework 3.0. LINQ to Objects for example relies on the new System.Core.dll containing the Enumerable
class. It is however possible to fake this dependencies and therefore use LINQ to Objects with the .NET Framework 2.0.
C# 2.0 introduces generics. This feature relies on extension of the CIL in the CLR 2.0. Therefore it is not possible to use generics with the .NET Framework 1.0 and 1.1.