Currently, we have 3 types of workflow in WF4; they are Sequence, Flowchart and State Machine. State Machine workflow is now CTP1, and will be published in .NET 4.5.
When your workflow include many sequence process, no much human interactivity, no much backwards. Use Sequence workflow.
When your workflow has backwards transition. Then use Flowchart.
If your workflow is totally dependent on the human interaction. Then, use State Machine.