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You are in luck, the iPhone 4S is compatable with iOS 8. In fact, it is the oldest iPhone hardware that will run iOS 8, according to Apple.
iOS 8 compatable iPhones: iPhone 4S, iPhone 5, iPhone 5c, iPhone 5s
iOS 8 compatable iPads: iPad 2, iPad Retina, iPad Air, iPad Mini, iPad Mini Retina
iOS 8 compatable iPods: iPod Touch Gen. 5.