Only 4 post on chrome box terminal?

The four pin is the newer design and it came about when they switched from the four pin dual ballast resistor to the two pin single ballast resistor. The pin eliminated from the ECU is the voltage sensing loop from the 5 ohm side of the old four pin ballast resistor - the new four pin ECU's no longer need it. A four pin ECU will work fine on the older five pin wiring harnesses, however, a five pin ECU will not work on a newer four pin harness as it needs that sensing voltage from the 5 ohm side of the four pin dual ballast resistor.