Hi @alex_aus ,
You may have to replace the screen/digitizer assembly to determine whether the problem is with the digitizer in the original screen assembly (which it may have been along with the cable as you couldn’t tell with a blank screen) or whether there is a problem on the systemboard or even that the replacement cable is faulty or the wrong one.
You did disconnect the battery before working on the cable etc, didn’t you?
Here’s a link to just one supplier It is shown to give you an idea of the cost of the part. There are other suppliers that may suit you better. Just search for Lenovo Tab 4 T-8504F digitizer to get results.
It may be possible to find a supplier that has a good returns policy, i.e. refund minus a small restocking fee perhaps, so that you can return the assembly in the event that it is not the problem, minimizing the cost to you