Workflow Foundation 4.0 Runtime will not host Workflows created in earlier versions of WF
Release 4 of the .NET Framework brings significant changes to Windows Workflow Foundation. The activities in the BAL have been re-written, for example, and some new ones have been added. This brings real benefits—many workflows now run much faster, for instance—but it also means that workflows created using earlier versions of WF can’t be executed by the .NET 4 version of the WF runtime. These older workflows can still run alongside .NET Framework 4 workflows unchanged, however—they needn’t be thrown away. Also, activities created using older versions of WF, including entire workflows, can potentially run inside a new Interop activity that WF in the .NET Framework 4 provides. This lets logic from older workflows be used in new ones.
link: http://msdn.microsoft.com/en-us/library/dd851337.aspx