Ecu 5pin's on system ballast 2 slots
The REAL question is, DO YOU REALLY HAVE a 5 pin ECU
Some 4 pin ECU's ONLY HAVE 4 pins
Some 4 pin ECU's HAVE ALL 5 pins. So get your ohmeter and probe the "5th" pin and see if there's any continuity anywhere.
Next to answer your question:
If you REALLY have a 5 in ECU, you MUST use a 4 pin resistor