Empty ProjectProperties4 and ProjectConfigurationProperties4 interfaces in VSLangProj90.dll ?
As you may know, while the general Visual Studio automation model uses the EnvDTE DLL series (EnvDTE.dll, EnvDTE80.dll, EnvDTE90.dll), there are some extension DLLs for the automation model of VB.NET and C#, the VSLangProj DLL series (VSLangProj.dll, VSLangProj2.dll, VSLangProj80.dll and VSLangProj90.dll). For more information, see my article INFO: Assemblies used in Visual Studio Extensibility.
Apart from some enums which are handy to interpret values returned from properties, the VSLangProj DLLs contain interfaces such as VSLangProj.ProjectProperties, VSLangProj2.ProjectConfigurationProperties2, VSLangProj80.VBProjectProperties3, etc. whose properties are the names that you can use when calling the EnvDTE.Project.Properties.Item("XXX") or EnvDTE.Configuration.Properties.Item("XXX").
However, if you look at the VSLangProj90.dll file of VS 2008 with the Object Browser, the VBProjectProperties4, VBProjectConfigurationProperties4, CSharpProjectProperties4 and CSharpProjectConfigurationProperties4 interfaces are all empty. I don't know the cause, but it seems an oversight, likely they were created and later it was forgotten to add the properties. If there were no properties to add, there was no need to create them, but certainly the automation model of VS 2008 has some new project properties such as "TargetFramework" that do not appear there and that I used yesterday in my article MZ-Tools Articles Series: HOWTO: Get the target .NET Framework of a Visual Studio 2008 project from a Visual Studio add-in or macro.