Can I use .NET 4.0 beta in Visual Studio 2008?

no, this is not possible, .NET 4.0 requires VS 2010 to be able to target/build.

I don't think there is - the IDE is usually tied to a particular version of the framework, and 2008 has been the first version to allow you to target a "non-native" (so to speak) version of the framework, instead of just the matching version.

While Microsoft might add support later, I wouldn't expect them to, though I would expect 2010 to be able to target different versions aside from 4.0 (I haven't used it, so it may do this and I don't know it). Update: According to this, 2010 targets 2, 3, 3.5, and 4.

To target 4.0, you'll have to use 2010 - at least for now, and probably indefinitely.

Some of new stuff that will be shipped with 4.0 were released as "extensions" and "CTPs", for example Parallels Extensions for .NET 3.5 will eventually be included in 4.0 but you can start playing with it now in VS 2008.

No, VS 2008 Multi Targeting option only support .NetFramework 3.5 , 3.0 , 2.0

so it has a Backward Compatability, So You have to Use VS 2010 for .netFramework 4.0