In the case of the "4 pin" ECU, the ballast is wired and does the same exact job as in a contact points system
In a "5 pin" (older) ECU, the twin ballast does the same job as above, and is simply TWO DIFFERENT resistors in the same mounting. The second half of the ballast --which is a different resistance-- supplies voltage to the "insides" of the ECU.
Chrysler modified the circuitry inside the ECU so that the second half of the resistor is not needed.
In other words, if you were to wire it up with the "twin" (4 wire) resistor, it would then work with EITHER Chrysler ECU
So:
Older "5 pin" ECU MUST have 4 pin "twin" ballast
Newer "4 pin" ECU CAN use either 2 or 4 pin ballast