@Arne-W just downloaded apm_planner_2.0.27-rc1-4-g562fa65_osx and it was able to see the PH4 and install the firmware without issue.
One very minor point.
On plugging the PH4 in during the FW upload (unplug and plug in instruction) the dialogue appeared about they’re being a new Plane FW version again. This was during the FW upload procedure so I am not sure if it would have interrupted the upload.
And after the verify and rebooting there appears an ‘unknown platform’ message in the output window.
It seems to be triggered when the serial port is selected again, ie: during FW flashing the serial port changes to some non existent serial device, and after flashing it reselects the serial port and generates the message.
If I choose another port and then the PH4 port again it generates the message again.