iPhone 5s or above with iOS 10 or above. https://support.pokemongo.nianticlabs.com/hc/en-us/articles/221958248-Supported-devices Up 0 Down
I checked that link too, but i saw some players in my area using iphone 4 to play the game just a few days ago. Up 0 Down
I have an iPhone 5s as a spare, works fine with POGO, visually resembles an iPhone 4. However, anything less than an iPhone 5s is just too little Up 0 Down
And if you want to use AR mode, I think you need at least the iPhone 8. Not critical, though, it's more of a gimmick. Up 0 Down