We are in the process of selecting a workflow solution for a company that uses Microsoft products end to end. Given the news on WF4, in that it seems to be essentially a rewrite
My understanding is that Microsoft will provide backwards compatibility and/or a migration strategy to the new WF, so I would guess that you are safe to use it. However, I have heard from other developers in my organization that the current version of WF is extremely painful to use. If you have the budget (and depending on the complexity of your workflows), you may want to consider K2: http://www.k2.com/en/index.aspx