Dlampe Dean from what you are saying, sound like junk in the carbs. If you are richening the carb for the one hole and the motor runs better there is something in the primary jet or holder that is cause less fuel than the cylinder needs. Sometimes just swapping carbs or injectors the crap in the carb or injector moves and stops the problem. If the carbs got to dry this is very possible to happen.
As far and the
oil in #4 sound like a leak seal or bent valve stem. The carb washes down the valve and you will see only a little
oil. If the spark is strong enough it will burn everything in the cylinder, air/fuel and
oil. The plug over time will look black but still fire. The other thing is the O2 sensor will get the wrong readings of the A/F mixture coming out the exhaust. The oil will slow the readings and say to lean out if the readings are low.
Try and replace just the valve stem seals for cylinder #4 and see what happens after you install new seals. Rick L. Ps you can do this in the car without pulling the head.