help me find an electrical gremlin, please

Time for my usual "lecture"

If you wire for a 4 pin ballast, you can use EITHER 4 or 5 terminal ECU

Of course a 2 pin ballast MUST have a 4 pin ECU

You can NOT tell many 4 pin ECUs by looking, as many have 5 physical pins. You have to use a meter to see if one is unconnected.