Once Apple insists that all apps become 64bit not 32bit how will this affect the app's performance on, and suitability for, the older iPads like my iPad 4 Retina)?
Will the iOS updates be downloadable to these older iPads?
iPad
If apple continues to make an iOS that is 32 bit then yes, if it's exclusively 64 bit the A6 chip in the iPad will not be able to run it. Apple is not company that historically divulges the nerd speak that we IT types like in lieu of adjectives and pictures of smiling people to describe your expected experience with their products.
https://support.apple.com/en-us/HT204204
Download iOS 10.0 - iOS 10.3 Information
Apple will be dropping support for 32bit at some point soon, sure (possible with iOS 11?). But developers will still be free to choose to maintain older versions of their software for older devices, as they wish to. Apple does not "instist" that they only code their apps in 64bit. They just won't be supporting it in further XCode releases as iOS won't support it. For those older, 32bit hardware limited devices, it will be up to developers to decide if they keep legacy support going for their apps, or not.
At some point, iOS gets dropped for all iOS devices, regardless. Your 4th generation iPad was discontinued in 2014, so at some point in the future, iOS will have moved on such that it no longer will support your hardware. That is a different issue though. When iOS itself no longer supports 32bit code, the iPhone 5 and 5c and older, as well as iPads (not Air, Air2 or Pro or the new iPad) and any devices with strictly 32bit processors, will stop getting iOS updates.
While it started doing so in 10.2.1, iOS 10.3 will pester you insistently whenever an ancient 32-bit-only app gets run, so you can help pester the developer to get with the program and upgrade their code. When the change finally comes with iOS 11 later this year, undoubtedly a large number of apps will be dropped. Their developers have fallen asleep at the wheel, gone on to other endeavors or might have even died. For these iPads and their corresponding model iPhones (5 or older), they will remain forevermore in iOS 10.3.x and their apps in the last 32-bit version released. In time dual mode 32/64 apps will disappear as well, depending on market conditions perceived by the developers.
Will the old iPads cease to work? Nope. Just like your Gramp's 60's Buick still runs. But it is not as fast, as safe or as efficient as today's models. And may be forbidden from entering some places. In the iOS word, this could be places like banks, secure networks, etc.
Older iPads - change from 32 bit to 64 bit apps
