Took a deep breath today and updated the GO 4 app to my iPhone (5s, iOS 10.xx) to v.22. My main GO 4 platform is now my new Mini 4, but I thought I'd try 'static' running with the phone before committing to .22 on the iPad. Then took a deeper breath and decided to just go ahead and update GO 4 on the iPad as well.

Problem: I got the AppStore 'update' notice on the phone, but none on the iPad (stayed v.20), so I flailed around finding ways to transfer the app from the phone to the Mini (the one I'd previously used using the AppStore and iCloud did not give joy) and found a way to delete/update it, but not transfer it from iPhone to Mini (this may all apply only to iOS 11.2.1):
Go into Settings on the iPad > General > iPad Storage and select the app to delete (dji GO 4 in this case). Tap delete, and you'll get the choice of deleting the app entire, or deleting only the app, leaving its documents in place (I'm assuming the docs include all the settings selected in the app, so decided to keep that for now; spent a lot of time on that, if true).

I deleted just the app ('offload app'). Then went back to the AppStore and downloaded the .22 version of dji GO 4, and I now have .22 on the iPad as well as the iPhone.
If I find that the settings are (mostly) untouched, I'll leave a note.
EDIT: Just started up a 'table run' on my Mavic, and it appears that all the settings I've input are unchanged. [bloody cold today here in the high desert for flying, but it's supposed to warm in to the high 40'sF for the rest of the week  'o)  ]

Might be of use. If there's a better way, please do reply.   

I never get the iOS update notice for the GO4 app on any of my iDevices. so when I comes out I just make sure all my records are sync. delete the app, reboot the iDevice and down load the new GO4 app. sign back in and resync my flight records. turn on the MP and check to make sure all my settings are where I want them. then do an slow and easy test flight. if all is at it should be, then and only then, turn it lose do the same thing on both my iPhone and Mini 4. Also i only do one iDevice and test it out before doing the other device. That way if i have problems with the update, at least i have one iDevice that will work until they fix the update.

Locoman Posted at 2018-1-5 10:41
I never get the iOS update notice for the GO4 app on any of my iDevices. so when I comes out I just make sure all my records are sync. delete the app, reboot the iDevice and down load the new GO4 app. sign back in and resync my flight records. turn on the MP and check to make sure all my settings are where I want them. then do an slow and easy test flight. if all is at it should be, then and only then, turn it lose do the same thing on both my iPhone and Mini 4. Also i only do one iDevice and test it out before doing the other device. That way if i have problems with the update, at least i have one iDevice that will work until they fix the update.

Totally agree. Why be totally grounded.