4 pin ecu used single ballast for the coil- ecu had full line voltage- if voltage surges- could pop the transistor/heatup.

dual ballast- same resistance on first ballast for coil--- second was for lowering the voltage to the ecu- hence- 5 pin.

you could use a 4 pin ecu in a 5 pin/dual ballast setup- but not a 5 pin to a 4 pin harness setup.