I just posted one entry in our "Go with the Flow" team blog to explain Workflow foundation V4's Activity Model. In the blog, I discussed how Workflow is composed of Activities and introduced basic Activity programming model in WF4. I quickly went over the 4 Activity base classes (Activity, CodeActivity, AsyncCodeActivity, and NativeActivity) for 4 different programing style (from fully decarlative to 100% imperative programing). I'll post another blog later for the same topic to give few examples on how to use those 4 classes.