Weak battery. Thought an iFixit replacement was in order
Looks like my 4S battery swap was not a total fiasco. The phone still seems to work. But I'm not seeing that much more life out of the iFixit swap-in battery v. the non-Apple battery that was already in the phone. I'm usually pretty impressed with the overall iFixit operation, but this one, not so much.
Take it very SLOWLY. There is nothing terribly complex going on here, but I really think the "guide" let me down a bit, around the part where the two screws need to be removed to detach the battery clip. There's a floating-around-gold/plastic piece that is pretty easy to trash. Apparently I somehow didn't, but I was left feeling like I dodged a bullet. And the iFixit "box of tools" turned out to be not that helpful either. Suction cup, really? The back of the case slides out pretty easily, about the only thing that was "easy." And I'm not sure I ever did get the included Philips head screw driver bit to work. I ended up using one from the my trusty iFixit tool kit. Could have been [probably was...] me, but my recent iPhone 6 battery replacement, with many more steps, gave me less problems.