After calibrating the Ph4's vision sensors through DJI Assistant 2, DJI Go App started to give me the error message "version mismatch. Update firmware" when I connected the Phantom 4 and ran the DJI Go App.
Following that, I manually initialized the controller to return the firmware to the factory default and then updated it to the recent version (v1.9 probably).
But it continues to give me the same error.
When I tried to calibrate the vision sensors, DJI Assistant 2 asked me to update the No Fly Zone database which failed after some time I started it. It justed said that "Update failed". I kept trying to update it but it didn't make through. I don't know the failure to update the NFZ could have inflicted that error.
I just want to know how I can get out of these troubles.
Thanks.

Hello and good day yowich. I am  sorry for the troubles that it caused. I would like to ask if you manage to update the aircraft and the RC to the latest firmware version? Did you try to uninstall and re-install the application? Please sync the flight records and save important files before deleting the app. Kindly give us an update for us to help you further. Thank you for the support.

DJI Stephen Posted at 3-2 15:06
Hello and good day yowich. I am  sorry for the troubles that it caused. I would like to ask if you manage to update the aircraft and the RC to the latest firmware version? Did you try to uninstall and re-install the application? Please sync the flight records and save important files before deleting the app. Kindly give us an update for us to help you further. Thank you for the support.

Hi, Stephen.
Thanks for the quick reply.
I had to calibrate the visual sensors with Assistant 2.
After I finished calibrating it, I clicked on Firmware update if there's anything I needed to update.
It asked me to update the NFZ database and I clicked Yes. But it kept failing to update it. So I gave up to do it.
I went out to the field for my work, but the DJI Go App showed me a message that "version mismatch. Update firmware."
I came back to my office and tried to update the controller but failed. So I reset the controller and updated to the latest version.
Then I tried to update the aircraft firmware. I don't know why, but I thought I had to update it to match the aircraft firmware to the controller's one.
But every time I run DJI GO App or Assistant 2, that NFZ update came up and asked me to update it.
I tried to do it, but it has failed.
I once had a chance to get into the firmware update menu in Assistant 2 without having a message asking me to update the NFZ database, so I was able to click on the aircraft's firmware update. But it failed as well after spending a substantial amount of time for downloading. I clicked on "Retry" to try that again but it starts all the way from the first beginning - downloading it. OMG! I think it will fail to update it again.

What do you mean by uninstalling and installing the application? Do you mean DJI Go App? or Assistant 2?

yowich Posted at 3-2 16:04
Hi, Stephen.
Thanks for the quick reply.
I had to calibrate the visual sensors with Assistant 2.

Have a look at This Post, the whole thread and another are discussing the very problem you're seeing. The post I highlighted provides a 'work around' until such time DJI provide a solution to the P4 flysafe database update problem that seems to be affecting a significant number of P4 owners. I've not yet seen anybody declare that the flysafe database update has been successful on the P4.

I seem to have found a temporary solution whilst waiting for DJI to get their house in order.  At least it has worked for me.
1.        If you don’t have DJI Assistant 2 install it from their website
2.        Connect the drone to your PC with the cable
3.        Ensure the drone and the remote controller are powered on.
4.        Open DJI Assistant 2 and wait for it to recognize your device and select it
5.        If you are prompted ‘Updating No-Fly Zone Database is required. Update Now?’ select ‘Ignore’
6.        I then selected ‘Restore Factory Defaults’ and waited for it to complete.
7.        I then selected ‘Refresh’ of the current installed firmware version and waited for it to complete.
And hey presto the drone is back to life!

WARNING – It will obviously prompt you to update the ‘Precise Fly Safe Database’ – CLICK IGNORE IGNORE IGNORE or you will be back to square one!

At first it will take a few minutes to lock onto GPS and will have reset any personalised settings, but at least it is back to life!

Hope this helps!

fans406434c5 Posted at 3-2 22:03
I seem to have found a temporary solution whilst waiting for DJI to get their house in order.  At least it has worked for me.
1.        If you don’t have DJI Assistant 2 install it from their website
2.        Connect the drone to your PC with the cable

Thank you very much Aardvark and fans406434c5.
I used Assistant 2.
I did the same way you said and that "Safe-Fly database update" message confused me getting into the firmware update.
Finally, I was able to get into the firmware update after ignoring the safe-fly database message.
But the firmware update also didn't go through well.
Lots of failures updating!
Then, I switched to DJI GO App by connecting it to the aircraft and made it update the aircraft firmware!
I double checked that DJI Go App didn't put that version mismatch message any longer.
But when I went out to the field to work, it gave me that same "version mismatch" message back again!

I had to cancel my trip and came back to try the aircraft firmware update through Assistant 2.
I found indeed there's the latest firmware that I need to update. Then why didn't that DJI Go App let me know the version didn't match? Weird!

This time, the aircraft firmware update went through well, and I was able to fly it.

BUT! DroneDeploy and PrecisionFlight were not able to do the planned flight!
Both apps connected well to the aircraft and then the aircraft was lifted to about 10 feet. But that was all; it didn't perform the planned flight. I had to land back again to check but found no issues.

I guess this issue should come from DJI Go App or Phantom 4 because both apps failed to do it.

yowich Posted at 3-6 10:48
Thank you very much Aardvark and fans406434c5.
I used Assistant 2.
I did the same way you said and that "Safe-Fly database update" message confused me getting into the firmware update.

I believe the flysafe database problem on the P4 was resolved by DJI a day or two ago, the latest flysafe database update seems to work fine.

Aardvark Posted at 3-6 14:18
I believe the flysafe database problem on the P4 was resolved by DJI a day or two ago, the latest flysafe database update seems to work fine.

I have just updated it following your message. Thanks for the information!