I've done the hop-on hop-off ticket twice now, and am still not convinced that it is worth it. You cannot possibly see everything in Vienna in 4 days. We took the two day ticket, thinking that we'd go back on our final day to see something we glossed over quickly. I think you need to be very organized to make these tickets make value-sense. Otherwise, you are better off just picking the sites you want to see and pay for them at the door
The ticket pick up process is not really flexible. We were already running short of time for 48 hours...when we arrived the palace stop the next morning, the tour guider insisted that we would need to go back to the subway station to exchange the ticket even though we had the print out and all documents. We lost the full morning sun. There should be an easy ticket pick up at the spot every bus stop. Also there were no ferries Wheel or Danube tower due to renovation.
Very good value and if planned properly it's possible to see virtually all of the sights Vienna has to offer. We took a 2-day ticket and had free entry to almost every place except the Opera House. Highly recommended.