That's right. Sony manual says 5th rung is non connect, but unclear whether they are talking about the headphone plug or the internal wiring of the jack. However, seeing as how Sony co-authored the spec, it would be strange that they don't have the full spec implemented on their DAPs. It would be very simple to wire the 5th rung to the common GND. Looking at the PCB image below, you can clearly see the 4 wires but there is also one more red wire that appears to be soldered to the 5th rung. I emailed the adapter maker on eBay so will share once I hear back.