I have found a very simple way to tell whether erroneous (re: wrong) touch response on an iPhone 4 or 4S is due to the Digitiser being damaged from a crack, or if it is a motherboard issue which is essentially unfixable (almost never is it the last scenario.)
In this image, you can see which has a defective digitizer due to a very deep crack which affects the LCD itself.
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If you see this rainbowing affect from a crack very deep into the display, and the digitiser is responding oddly, then your motherboard is fine, and a new LCD/digitiser assembly will fix the issue.
In this image, you can see which has a defective digitizer due to a very deep crack which affects the LCD itself.
If you see this rainbowing affect from a crack very deep into the display, and the digitiser is responding oddly, then your motherboard is fine, and a new LCD/digitiser assembly will fix the issue.