We just got our S4 and are loving it. However there's a feature request I think would greatly improve the user experience.
Silent operation when instructed to go to clean any room away from the charging station.
Once arriving at the first room then start the vacuum motors, but not until then.
e.g. If you store your robot in your bedroom (like we have to) then it would mean that we can schedule the robot to clean other rooms at night so not to disturb our sleep. The robot could then be scheduled to clean the bedroom later once we've moved to the main living area.
This way the robot can quietly make its way out and begin cleaning.