Try with Assistant 2, with DJI supplied cable, but first I'd uninstall DJI Assistant, and re-install it. Did you try doing the update with MP powered and linked to remote and attached USB to remote to computer? like here:
You can also try this: If you go on the first splash screen on DJI GO4 where it has a picture of the mavic, in the top right corner there's three small lines, if you click it shows options for flight record, academy and store, instead of clicking, hold it down for like 5-10 seconds, when you release it will show firmware, select to download, then try installing it that way.
You could also try downgrading firmware via Assistant 2 and then try again.
All else fails, try sacrificing a chicken, on a full moon, while doing a compass calibration...
I hope you get it sorted, but you may need to call DJI support.
Really appreciate the help here. So far, no go - but I'm pulling out all stops.
I plugged in each device individually to a pc (assistant 2) next I downgraded and refreshed to 1.3.000
Then a new battery, and onto an Ipad Pro - reinstalled that app, connected the RC to the IPAD and connected to the mavic (so they are both fully fresh on the 1.3.000) Now I am starting the update to 1.3.4 again.
WISH ME LUCK. Next I'm going to find a chicken.
I am using RC to communicate, does it matter?