Can I develop for .NET Framework 4 in Visual Studio 2008?
You cannot use Visual Studio 2008 to develop 4.0 applications in a supported manner. Currently only Visual Studio 2010 can be used to develop 4.0 apps. Starting with 2008 you can use it to develop for earlier versions of the framework but not later ones.
From ScottGu's blog:
There isn't any way to target .NET 4 from VS08 and use new features. Having said that, .NET4 is upwards comaptible with .NET 3.5 - so applications you build targeting .NET 3.5 with VS08 should work fine on top of .NET 4.
The only way to develop for .NET 4 is to use Visual Studio 2010. However, if you do create a managed assembly in Visual Studio 2008, you could run in on a machine with .NET 4 run-time installed.