Exactly. The four hours count as both sleeping and resting. So, it takes D6 hours to get the first point of Wits back (and get back into action, if needed), and then another four hours of sleep to get the rest of your Wits back. If you want to minimize the time out of action you can postpone the four hours of sleep to a later time. Same thing with the (other) four hours of minimum daily sleep - these periods of sleep / rest don't need to be consecutive, you can do other things in between.
