Compression,
Granted this has been ruled out since you tested to ground, but if checking across the plug gap a higher compression will show a better spark at the wire since it is a harder spark in the chamber. This would make #4 show a weak spark at the wire since it requires less voltage to spark in the lower pressure.
ECU,
The ECU has a single driver circuit triggering a single ignitor near the coil. I do not envision how this could be selective to a particular cylinder especially where the hall effect unit in the dizzy has been swapped out. That reduces the chance of an out of round trigger giving an asymmetrical signal especially since the ECU is not sensitive to the hall trigger signal duration for setting dwell.

I expect this is not a smooth running engine for reasons other than a spark related issue. I myself would swap it out for a younger one.