The power switch on the iPhone 4 was not working anymore. I had already opened it up once to repair the battery and at that time, I was able to see the broken switch.
The screws are TINY! The screwdriver that iFixit sent me was completely useless, it didn't fit a single screw in the entire phone. Luckily I had another screwdriver at my house that, although slightly too big, was workable.
This repair is tricky. Not only are you working at small scale, but lots of pieces are taped or glued together, and pieces overlap so carefully that you have to be very careful about what order you take things apart and put them back together. At several points during reassembly, I had to backtrack in order to fix a slightly askew part that had several things layered on top of it. Patience is a requirement for this repair: this took me about 2 hours from start to finish.
Almost none of the screws are the same size, so I recommend that you carefully keep track of which screw goes where.
I looked for an iphone 4 video on youtube. I found that to be a little more useful than the pictures that iFixit provides. You need sharp eyes, a steady hand, and comfort dealing with tiny electronics. This repair is not for the faint of heart.