Did you install a separate version of the HSS shader? It's included with DS4, so if you did you may have repalced a working copy with a non-working copy. If that isn't it, do a render, then go to Help>Troubleshooting>View Log, scroll to the end and see if there are any shader-related errors. Also, make sure you are using the 3Delight render engine (top poisition on the quality slider in Render Settings).