Version 4.0 of DotNetNuke is built against ASP.NET Framework 2.0 and using Visual Studio 2005. We have to rebuild the dll's using the VS 2005 which will reference the 2.0 dlls instead of the 1.1 version. Additionally, if you see the post which Charles made, the structure has also changed in 4.0 which will be different than the 3.2 version.