Workflow Foundation .NET 4.0 not quite the cousin of Workflow Foundation 3.0/3.5
Check this out, it appears that WF 4.0 is a completely new framework, not related in any way to WF 3.0/3.5.
Good news is that you can still use old 3.0/3.5 activities as long as you wrap them in a interop activity.
Now this is not really bad news, reality of the situation is that, albeit I (think I) am maybe one of the biggest fans of WF (and many months ago this was my first .NET 3.5 certification) and there is quite a bit of us out there, I found that most junior and intermediate developers find it difficult to understand and they're quick to give up on it from the start, even some (not just a few) senior developers seemed scared of it.
This implies that curent version of WF is not the most approachable framework, and good people in MS decided to do something about this, I expect WF .NET 4.0 to be much more approachable and more RAD, so lets see how it plays out.