The connectors are the same for the digitizer and LCD and the screen will function properly between the models but the parts are physically different and will not fit either way. The sensor layout is also different which would cause the phone to not work properly. The home button is also different.
In short, you can't do it.
You say the parts are physically different, would you please elaborate on that? I've read another post here on iFixit implying that at least the LCD can be interchanged. It would be nice to have a post here on iFixit to refer people to that explains the differences
The LCD and digitizer are fused together on the iPhone 4S still just like the iPhone 4. The LCD cannot be separated from the digitizer without breaking one of the components. That means that you cannot replace just the LCD or just the digitizer individually. You have to replace the entire assembly. Like I said above, the connectors for the LCD and the digitizer by themselves will work on either phone. But you would not be able to physically install the part because the bezel around the assembly has a different layout when it comes to where it screws into the mid-frame. The sensor window is slightly different as well so the ambient light sensor and proximity sensor may not work properly.
I have been replacing iphone screens for a couple years now. 3G 3GS 4 and 4S.
I have had to substitute a 4 for a 4S and vice versa a couple times. Of course, it is best to use the proper model whenever possible, but here's a trick if you absolutely have to use the wrong screen [and therefore the wrong bezel].
Chopping off the brackets can be very hard to do unless you have the right sort of metal cutter / industrial scissors.
Instead, you can carefully remove the bezel from both the cracked (original) screen and the new screen and reattach the original bezel to the new screen.
A couple things to be careful of:
If you pull too hard on the bezel, you will snap it / crack it. Not the end of the world (you can still use it), but be slow and deliberate when pulling away from the screen/LCD.
Also, if you pull too hard and with too much force, you might crack the LCD and have the dreaded bleeding ink drip look negating your whole endeavor.
Carefully start by pulling on a corner with your thumb pushing the screen away and your fingers pulling toward you on a corner bracket.
Make sure you pull the sticky tape along with the bezel and keep it in place as you go (it will try to pull away). This is extremely important as that sticky tape is the ONLY thing holding the screen to the rest of the body.
This is a drastic step, but it does work.
I know I have replaced 4 screens with 4S without a problem. I can't say with confidence that it works the other way around.
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