Hello,
Newbie here.  

Issue 1 - DJI assistant 2 doesn't recognize my P4.  I've downloaded the software and installed/uninstalled it per instructions several times.  I've read through other posts and tried everything suggested but still no connectivity.  I'm using a Windows based PC.  I was however able to upgrade the aircraft firmware via the DJI GO 4 app on my iPad.

Issue 2 - After the firmware upgrade, I get a "Version Mismatch.  Update all modules" error message.  I only have 1 battery and performed the firmware update per instructions.

Any help would be appreciated as I've yet to fly this thing....

Hello and good day Jacabal. Welcome to DJI Forum and 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?  For reference I will be posting a link where in you can find the most recent firmware for the DJI Phantom 4 Advanced. Please try to uninstall and re-install the application once again? Please sync the flight records and save important files before deleting the app. Kindly keep us posted on the latest progress. Thank you for the support.

https://www.dji.com/phantom-4-adv/info#downloads

I seem to have found a 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!