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I think they will as soon as the 5s becomes outdated enough. It is still a good phone, but already lacks new hardware features like Pay. The demand for the 4" screen is there. They should discontinue the 5s next year and replace it with an updated 4" screen 6s with all the new stuff. IMHO that is.The 5S is an awesome phone and still very current. You can get a great deal on one now too. I'm not sure Apple with release another 4" screen iPhone again. Time will tell.
Maybe Apple will release an iPhone 6c with a 4" screen.