The wiring is as follows:
I used a standard 3.5mm, three conductor inline socket and a 4.5mm four conductor plug which Jaycar carries in store. Doing it this way means I could leave the Mic PTT switching turned on, but I use the transmit switch (XMIT) on the lower front of the rig for control. Here's how it looks:
When I first tried this, mic gain was way too low, but I noticed that Elecraft had a firmware update I hadn't applied that offered increased microphone gain. "SUPPORT FOR HEIL HC-6 MIC ELEMENTS: Mic gain can now be set as much as 20 dB higher to support these low-sensitivity elements."
On air reports from Mal, VK2BMS and Stephen, VK2BLQ suggest that the audio from the $15 headset is a little lacking in bass compared to the normal microphone but this can be adjusted if required. The headset is very light and comfortable but probably not robust enough for field use.
Incidentally, I accidentally interrupted the firmware update process and this left the KX4 in a mode where it displayed "MCU LD" on the display with the red TX light flashing. I wrote to Elecraft customer support and Howard replied within minutes with the procedure to get past this. (I won't reproduce it here as I guess it might change over time).