I can't download the firmware using either the Go4 App or DJI Assistant. It downloads to approx 90% and then gives an error message that update failed. I just purchased this from a private seller, so I don't know if it is something he did or another problem. I have also tried refreshing the current update, which was also a failure. Any help would be appreciated.

I am also having this problem downloading the latest and also previous firmware (my current fw is V01.04.602).

I have tryed via mobile app (DJI GO 4 for iOS) and via DJI Assistant 2 for PC.

With mobile app - it pauses the firmware download at 91% and can't go further. I can retry forever and nothing happens.
With DJI Assist 2 - i got the same error as you on 90% of the firmware download. I don't know how to check what the error is.

I have also tryed to reset the drone to factory settings via DJI Assistant 2 and then upgrade the FW but it got me the same error.

Have you tried to contact DJI about this ?

If you contact what did they say ?

I have wrote an email to the support and had 2 times chat with the support... no results and no answer to my email.
“ Your request (2738478) has been received and we will contact you as soon as possible.”
Meanwhile the problem is still present... at least in my case

In another post i have published the reason for the problem. I will do it here also:
I think it’s related to DJI server side.