All ECU plugs are ready for 5 pins, no one for 4 pins. If the harness is set for single ballast, then the unused pin wire is arriving nowhere down the harness.

Now, if the harness is set up for dual ballast, then the 5th pin wire (green with red trace) is arriving to the secondary resistor section. Which still if using a dual ballast with a 4 pins ECU, it won’t be any kind of load (althought still voltage present) on ballast secondary resistor or related wiring. Just with 5 pins ECU it will be in real use with some load.