I just spent the whole day on a windows 10 PC trying to get the firmware to update as above uninstalled reinstalled tried and tried 20 or more times.
To cut a very long story short I was successful after fully charging all batteries all controllers all interface iPads. Also uninstalled go 4 apps and reinstalled them. Uninstalled Assistant 2 downloaded the latest and reinstalled it.
Used Assistant 2 on a windows 10 PC first to update/reinstall each controller firmware. Ignore any windows 10 messages saying a drive needs to be fixed. Next used Assistant 2 to update/reinstall each
M2P(3 of them) to the latest firmware.
Charged all the batteries again. With a newly installed go 4 app connected to each drone in turn with the same battery and reinstall the latest firmware with success.
Next using the same drone switch batteries and using Assistant 2 reinstall/update the firmware.
The short answer is to uninstall, download and reinstall all software(Assistant2 and DJI go 4 app) use Assistant 2 first on individual pieces of kit before trying using the go 4 app. Ignore any windows 10 drive error messages about LOST.DIR folders.
Best of luck!