That is for all 4. They charge by the bike- not by the person. We rented one a few years ago. When we rented, the attendant said, "Take your time." I took that as "If you take 45 minutes or so, we won't charge you more." I was wrong. It took us more than 30 minutes to go around the loop (we stopped a couple of times), and we were charged for the extra time.
Just be sure to get a clear answer from the attendant if you will be charged for having the bike out more than 30 minutes.