Put a stethoscope on the injectors of 2 and 4. If they click then you know they are at least being triggered. If not, then you should start looking at the ECU if you confirmed the harness is good to the ECU connector.
Next you could get a spark tester and put that inline and visually inspect the plugs sparking.
You already said you had compression. If your numbers are in spec and have good uniformity within 14psi of eachother then it's not likely that your valves are stuck. If in doubt still, do a leak down test on all cylinders.