Press the rear light.  Is it a button?  What's it for?  Hmmmm...

It is a button. According to the manual, it is labeled - Aircraft Status Indicator/Function Button.  And the description  in the manual says - Performs various functions based on settings in the DJI GO 4 app. Best I can tell from reading the manual, it's used to reset the WiFi SSID and password and to switch between using the RC and your mobile device and for linking the remote to the AC. That's all I could find.

Thank you, end thread.  Ironically RTFM.

FatherXmas Posted at 2018-2-9 17:20
It is a button. According to the manual, it is labeled - Aircraft Status Indicator/Function Button.  And the description  in the manual says - Performs various functions based on settings in the DJI GO 4 app. Best I can tell from reading the manual, it's used to reset the WiFi SSID and password and to switch between using the RC and your mobile device and for linking the remote to the AC. That's all I could find.

Wonder if this is the button to start capture mode without RC as mentioned in today’s FW..

Press 2 seconds for RC mode
Press 4 seconds for direct phone connection (WIFI mode)
Press 6 seconds to reset the WIFI SSID and Password for direct phone connection

https://forum.dji.com/thread-132494-1-1.html

It really is scary how many people don’t bother to read the manual

Kevin79 Posted at 2018-2-9 17:51
It really is scary how many people don’t bother to read the manual

Can't disagree with this.  I missed it.  My bad.

Yep, it's an aircraft indicator and function button. Way to the manual, click this.