With the iBOLT Dock'n Play cable the S4 will automatically go into "DockMode" (it will say "The Dock is connected" at the notification bar) and the sound from Apps will be routed through the microUSB cable if you go into "Settings-Accessories" and select the "Audio output mode". I made a new video today showing how this work, see https://www.youtube.com/watch?v=yWWz6g9XjUw& I will also subscribe to this thread so I can answer any technical questions coming up.
When your Galaxy S4 is in DockMode you can also select a "Carhome screen" so the Bluetooth an WiFi turns on/off automatically. Two great apps are the newly updated Dock'n Drive (free App with no annoying advertising) that now features a floating icon when in DockMode so you can quickly go back to the "home screen". Another very good App is the Ulysee Speedometer for home screen when driving.
The cable is 9ft long so anyone in the backseat can stay connected and charge or if you use a car-dock you can place it on the left side of the steering wheel and run the cable down the drivers door molding and under the floor mat to the center console where power and aux-in is usually located. Price is only $14.95 (lowered from $19.95) but for Memorial day weekend the code "got Memorial Day?" knocks of an $10 additional on any product off the iBOLT website off so you get by and pay an quite unbelievable $4.95 for the Dock'n Play cable.