Hey!

DJI specs on the phantom 3 pro indicate that you can record in 4K in either 4096x2160   OR  3840x2160
               The specs read:     UHD: 4096x2160p 24/25, 3840x2160p 24/25/30
Does anyone know how to choose between the two options? Best that I can tell in the video settings in the app, you really only can choose 4K, but not the sub resolution

You just tap the 4k resolution icon you want, you should be able to choose both options. If you still have issues, then delete and reinstall the app. If you do, make sure to save your flight records to the cloud.