How can I permanently delete personal data from iPhone before selling

I have a old iPhone 4s and want to change a new iPhone 6s plus, I want to sell the old iphone 4s but I really worry that some technical guy could retreive my sensitive data from my iPhone although I 've delete them,# can my data like messages, contacts, photos be easily recovered by data recovery app?# how can I do to make my data never be recovered?

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