Well to start narrowing it down, I would switch the heads. ie: take them off the engine, and swap banks with them. Then if you start fouling #4 instead you KNOW your problem is in the head. If the problem is still #5, then it has to be rings.
When you R/R the heads last time, did you put them on the same side? Were they marked at all so you knew which was which? How did that cylinder wall look when you had the heads off?
I'm gonna bet you have a BROKEN piston ring, not necessarily a worn ring.
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