Will iOS7 apps run fine on iPhone 4? Some apps like Evernote and Superevent require iOS7, and have been "optimized" for iPhone 5 and above. Will such apps work fine an iPhone 4 that has been upgraded to iOS7.1? On a forum I heard that upgraded iPhones have "missing frameworks" or something, so I don't want to buy this iPhone if I cannot run Evernote and Superevent.
"Optimized" apps are adapted for the bigger screen sizes of later iPhones, they may also include functionality which requires new hardware only available in these models (e.g. for Touch ID). Besides that apps which run under iOS 7 run on all hardware supporting this iOS version.