4 pin vs 5 pin?

In a nutshell... The 4 pin ECU can use the single ballast resistor.

The 5 pin used the dual resistor. One side of the resistor (.5 ohms) was used in the starting circuit. You would turn the key to "start" and the electrical current would flow through that side of the resistor. The other side (5 ohms) was used in the run circuit. The car would start, you would release the key and the electrical current now flows through the 5 ohm side.

The 4 pin has no current flowing thought the ballast when the key is in the start position. In the run position, current flows through the 1.2 ohm resistor.

EDIT: corrected resistance value of dual ballast resistor