I have the same problem now: At first my touchscreen reacted only if the keyboard was retracted. After one more day it doesn't react at all anymore. I searched for a new flex ribbon cable, but wasn't sure, how the LCD is connected to this flex ribbon cable, so I didn't bought it yet. As I was determined to even kill the Droid 4 due to the malfunction, I disassembled it again, carefully pulled the black tape off and tried to analyse how the connection is established. As the flex ribbon cable seems to be broken in my Droid 4 I even pulled off the two cables from each other. It felt like some sort of adhesive tape, which is glued to another tape. 25 PCB tracks and contacts on 7mm were revealed. No soldering! If I try to put the two just separated cables together, they are not sticking together again. There is no adhesive between them. Due to the REALLY small contacts (approx. 4 contacts on 1mm) there is absoluelty NO way to connect both cables together again, unless you have very good tools (i.e. microscope etc.). So, the only solution is to buy a LCD incl. flex ribbon cable.
Does anyone know more about this connection? I'm really curious how this connection is made...