5 - 30 minutes
The power/lock button stopped working and also the phone was barely able to stay alive for 15 minutes.
I followed the repair guide and by and large the repair went very well. I skipped steps 20 and 21 as it is not necessary to remove the front facing camera for this repair. When I got to the power/lock button, I couldn't see any difference between the new one I bought and the one that was in the phone. However, I continued and replaced the old button with the new one anyway. I tested it once I reinstated the two 1.5 mm Phillips screws securing the power button bracket to the outer case, but it still did not 'click' or even move when I pressed the button. On further examination, it seemed that the problem was that the button does not make contact with the actual physical button on the securing bracket. So I removed the bracket and then the button. I decided to modify the button so that the small circular disc inset into the button had a bit more height. To do this I decided to use a two part epoxy glue (Araldite), of which I applied a very small amount with a tooth pick. To prevent the drop spreading out and flattening, I place it right-side up (i. e. glue side down) on two toothpicks until the glue had set. This worked well and it seems that you only about 1 - 1.5 mm of additional height. I would say that the correct part to replace would actually be the Power and Sensor Cable. I can't see how this fault would ever be the button, although replacing then Power and Sensor Cable is a bit more work. Anyway, I was happy with my fix and it seem to do the trick. From here I reassembled the phone following the guide in reverse and installed the new battery when I got to it. Completed reassembly and then plugged into the charger to charge up the new battery. After a few minutes the Apple logo appeared so I felt it was good. However, after the phone was on charge for a few hours, it did not boot to the home screen. Every time I removed the charge cable and re-inserted it, the logo would appear then disappear but would not boot up the the home screen. At this stage I was feeling uneasy and wondering if I had bricked it. But I decided that I would not panic yet, and as it was late, I would take a break and get some sleep then take a fresh look at it in the morning.
So after a good sleep, I decided that most likely (or at least, I hoped) I hadn't attached one or more of the cable connectors properly. So I decided to disassemble up to step 19 - removing the logic board. I then reassembled, taking a bit more time getting the logic board in place and this time it felt 'right'. I also took more time reattaching all the various ribbon connectors taking care to align them and ensure they snapped in place. I finally got the two screws by the dock connector back in. Turned it over, and voila! The home screen!.
I was so relieved. A successful repair.
The phone is going great, and battery life is like new.
Take your time. Read the guide several times before you start. Watch the video a few time as well. Read all the comments relating to each step, which are after the last step of the guide. The steps where there are more than one screw of different sizes, put each sized screw in a separate cubby and in the order that they are mentioned in the guide. If your phone does not work at first don't panic. Calmly redo the disassembly and reassemble paying more care to ensuring the connectors all snap in positively.
Anti-Static Project Tray
iFixit Opening Tool
iPhone 4 Replacement Battery
iPhone 4 and 4S Power and Lock Button