The IPhone 4 battery had started to lose its Mojo. Wouldn't hold a charge for more than a few hours. Well, what do you expect after 4 1/2 years.
So I pickup up the Ifixit kit from the mail and immediately head to the gym where my son and I are going to play racquetball. After the game, I pull out the kit and say "Now, when you get home unscrew the two screws ..." And I'm about to say "make sure you are on a good surface, and go slowly..." When he already has the screws out and is sliding off the back. Then I say "Now you might want to go slow prying the old battery out, when POP ... out goes the old battery. (Then he realized that he should have unscrewed the electrical connector.) ZIP, out came the connector, in went the new connector. Slide on the back, he elected to use the "liberty" Phillips head screws, spin spin. It was done. At a table in the gym, in about the time it is taking me to write this. Next time, with a stopwatch, I'll measure it. It can literally be done in one minute. He let the battery run down to zero (it was at about 40%) then charged it. His phone is like new from the box. $25 or so. Happy son, happy father.
Don't be afraid. But do be on a table that will catch one or more of those tiny screws because it they ever hit the carpet, they are gone forever.