These are the parts you will need:
Rear Panel, Front panel (display assembly) and either a replacement power button and sensor cable assembly OR if you have fine tweezers / razor blade you can replace this part on the power button instead:
Okay so what you will want to do is first disassemble the phone to where you get at least the back cover, logic board and front display assembly out.
Guides to replace parts:
Rear Panel Replacement: iPhone 4S Rear Panel Replacement
Display assembly replacement: iPhone 4S Display Assembly Replacement
Power and sensor cable replacement: iPhone 4S Power and Sensor Cable Replacement
If you want to buy the parts from iFixit:
Rear Panel Replacement: iPhone 4S Blank Rear Glass Panel
Display assembly: iPhone 4S LCD Screen and Digitizer
Below is very lengthy, please bear in mind:
If you do choose to buy the small yellow sticker with the button on it to fix the power button (personally I find it easier to do than replace the whole part which takes longer) you will need to use something like fine tweezers, razor blade or exacto knife works too.
Basically what you do is get the phone disassembled to where the front screen is removed. Then remove the two screws that secure the lock button under it.
Then loosen the power button assembly and carefully remove the yellow sticker on the home button mechanism itself.
Take note that there are like two metal pressure plates, one will stick on the yellow sticker itself and the other will fall right off and be hard to find if you don't take note of it.
The one that is loose you will want to keep. Once you have removed the yellow sticker from the metal plate take the other yellow sticker from the other replacement home button. Make sure to place the loose metal pressure plate onto the power button first then put the replacement yellow tape with the metal pressure plate stuck onto there.
If it is done correctly you should have placed the metal pressure plate on the power button then the yellow tape replacement power button on top and made sure it is secured on by pushing it down carefully on the yellow tape around the power button pressure plate with fine tweezers.
After you done all that simply replace the power button back to where it was secured, don't forget the large metal physical button to align back in place too before putting the two screws back in. And it should click again after that.
It's a common issue on iPhone 4 and 4s series phones where the small circle plastic part above the power button gets worn out over time.
Then you are ready to install a new front screen and rear panel after that. It's best not to repair power button when a screen is installed as you can risk bending and damaging the screen cables.