I fix Macs..for a living...
iPhone 4 Battery
I replaced the Home button and the Power & Sensor cable, along with battery. Let me explain some caveats with ifixit's parts (OEM Apple parts):
-The power-sensor part does not include a new gasket/filter for the sensor- recommend tweezers and care.
-The power-sensor cable needs the foam pad transferred with the adhesive- extreme care with removal.
-IF the power button isn't working it is likely the cable, not the button.
-I used a small ice cube tray.
-It took me an hour (I had to get up and walk away...frustration factor high with the delicate flex cables and adhesive). Lit magnifier is welcome. Definitely use a spudge stick.
-I broke 2 of the 4 shield/antenna tabs that you need to tuck down from the speaker on re-assembly. I didn't force them, but so easy to press too hard in seating the speaker.
- I do repairs on a grounded-ESD mat and wore wriststrap.
-Right screw on the power button brace is PITA to access...careful pressing on the display flex cables to access the tiny screw. Magnetize your bit!