Are there any iOS devices that run iOS 7 but won't be upgradable to iOS 8? I'd like to know if I need to keep a iOS7 device laying around for a test case or if ultimately every iOS 7 device will be upgraded to iOS 8.

I have several iOS 6 devices that cannot be upgraded to iOS 7.
As far as I know, the iPhone 4 is the only device capable of running iOS 7 that can't be updated to iOS 8.

Ref: Wikipedia - http://en.wikipedia.org/wiki/History_of_iOS#iOS_8 - &amp; others