What part do I replace for a sleep button that won't click?

Buy a new cable.

I repaired a power button on a 4 for a client who had dropped her iPhone on the button itself and it no longer "clicked", and it also cracked the screen. The button on the inside of the phone should have a tiny orange "nub" on it which produces the click. Sometimes they fall off so I removed the cable with the button assembly and saw that hers was still there, but it would not work. Also the assembly appeared to be bent at the midpoint so I straightened it best I could and put a couple layers of tape on the button to build it up a little more. I used a hole punch to make the tape pieces round, and they fit just right. After reassembling, the button worked satisfactory.... (iPhonesfixed)