As an update on this thread we opted to leave #4 in place for now as it is working and not leaking -- live to fight another day so to speak. They were prepared to do something like (if not specifically) the heli-coil but the general position of #4 and the difficulty in actually removing it made them pause. They seemed to think it would be very hard to remove - too hard and too costly.

89 Raider V6
95 SR Montero