I am back, sort of. I am currently on the SketchBook QA team and we are getting a lot of tickets on this topic so I wanted to share this with the community.
If you get this error when trying to update SketchBook from 3.7.4 to 4.0.4 on an iOS device: This app is not compatible with your device.
DO NOT DELETE IT FROM YOUR iPad OR iPhone! If you delete it, you will not be able to re-download it from the AppStore because the version on our AppStore is 64 bit. Believe it or not, there are iOS devices are 32 bit or 64 bit and the easiest way to tell is by processor type. You just need to know this one rule:
Any iOS device with an A7 Processor or higher is 64 bit.
We have a long list of compatible hardware and it is noted on the SketchBook iOS product page:
Compatibility: Requires iOS 9.1 or later. Compatible with iPad Air, iPad Air Wi-Fi + Cellular, iPad mini 2, iPad mini 2 Wi-Fi + Cellular, iPad Air 2, iPad Air 2 Wi-Fi + Cellular, iPad mini 3, iPad mini 3 Wi-Fi + Cellular, iPad mini 4, iPad mini 4 Wi-Fi + Cellular, 12.9-inch iPad Pro, 12.9-inch iPad Pro Wi-Fi + Cellular, 9.7-inch iPad Pro, 9.7-inch iPad Pro Wi-Fi + Cellular, iPad Wi-Fi (5th generation), and iPad Wi-Fi + Cellular (5th generation).
For more information on 64 bit support on iOS from Apple, go here:
If you need to backup your older version of SketchBook, you can back it up via the iCloud.
I have also talked about 32 and 64 bit computing back in 2011 from this older Up and Ready Blog post: