5 pin ECU vs 4 pin

^^Right. ^^ 4 pin ballast and harness will work with either 4 or 5 pin box

A 5 pin box MUST have a 4 pin ballast

You CAN NOT identify 4 pin boxes easily because many (most?) have 5 physical pins. You would have to do resistance checks from the 5th pin to see if it is connected. I don't know what the last Mopar box 5 pin was made, but MOST OEM and ALL aftermarket boxes you encounter are most likely 4 pin.

Another way 'to tell' would be to disconnect the box side of the 4 pin ballast when running. If it keeps running........it's a 4 pin box!!!!

On a side note:

The "coil" side of the 4 pin ballast is just like any 2 pin as with breaker points, etc. In fact if the box quits, and you have a points dist. You can just unplug the ECU, drop in the dist, connect the dist. wire to the NEG side of the coil and "off you go."