The iPad 2 has been the bottom rung for both iOS 6 and iOS 7, and may well be removed from the list of iOS 8-compatible hardware (especially as Apple no longer sells this tablet). Our prediction is that iOS 8 will be compatible with the iPad 3 and later. Update: nope — the iPad 2 survives.
It's possible that the iPad mini 1 won't be able to run iOS 8 but, with the device still on sale and only two possible mini models for buyers to choose from, we think this is unlikely. Update: correct.
iPhones have been moved downwards on the iOS ladder at the rate of one model every one to two versions of iOS. Which suggests that the iPhone 4 might survive, and it might not (it's only been on the bottom rung for a single version). But given the performance problems that have beset iPhone 4 models running iOS 7 (slightly alleviated, admittedly, by iOS 7.1), we think it will almost certainly be removed from iOS 8 compatibility. Our prediction is iPhone 4s and later. Update: correct. Three out of four, then.
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