I’ve discovered recently myself that there’s not a lot of online content available. Check out the Workflow Foundation 4 Resources and Community site on MSDN for some good articles and labs. Also, download the WF4 Samples. I’ve learned a lot from poking through these samples and trying things out locally.
As for the WHY of Workflow, you can read David Chappell’s take here. I don’t agree with some of what he claims (a bit too pie in the sky for me), but it’s a good primer on the goals and rationale of WF.
Hope that helps!