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I used the fact that my beloved 4s had a cracked screen as a reason to get a 5s; my wife then noted that the 4s had double her iphone 4's capacity.
I was flanked by my sons, who were ready to see papa get his fixin' geek on. When we discovered that replacing the screen meant basically eviscerating the whole phone, they wandered off to play Minecraft...but I persevered!
The process took about two hours, and I'd recommend having a safe place to store lots of TEENSY screws, separated by step (maybe 12?) somewhere unlikely to be scattered by a bump from the arm.
A few of the corner screws are hard to get to; be ready to hold the driver at an odd angle.