I tried to upgrade my firmware today using DJI GO 4. The aircraft update ran through to 99 percent. The screen said to disconnect cable and reconnect did so but the update never completed. I restarted the remote and aircraft. When I restarted the app the firmware for remote did not download and it says firmware version for aircraft is unknown. What should I do. The light on my remote is red and flashes green.

I am having the same issue.  I called DJI support and they have a recording up saying they are having technical difficulties at this time.    There server must be down

I would try again using DJI Assistant 2 which you can download from Here