As you've probably already heard, Microsoft released .NET 4.0 Beta 1 on May, 20th.
SharpDevelop 4.0 will be the SharpDevelop version built on top of .NET 4.0. I've just got it running:
Yes, that message really reads: "Compiling is not yet implemented". There were huge changes in MSBuild 4.0 and the parts of SharpDevelop's project system that are talking to MSBuild will have to be rewritten.
The .NET 4.0 work on SharpDevelop is going on in the dotnet4 branch, for which we do not provide builds. The dotnet4 branch will be merged back into trunk as soon as it's good enough so that I think other SharpDevelop contributors can be expected to use it.
But myself, I'm currently stuck using a dysfunctional version of SharpDevelop for development. I cannot use previous versions since they cannot compile for .NET 4.0 and don't have code completion for 4.0 libraries. Using the dotnet4 version of SharpDevelop at least gives me code completion for 4.0 libraries, but it looks like I'll have to compile from the command line for some time.
I cannot even use Visual Studio 2010 as it doesn't want to open ICSharpCode.SharpDevelop.csproj - it says it's an unsupported project format. Even if I recreate that project in Visual Studio, VS doesn't want to open it - looks like a VS bug to me.