I have been reading back to 2006 forum when members had issues with plugs not firing. My #4 stopped firing yesterday and I have checked plug wires and possible wire grounding. I am running petronix ignition with an MSD coil and Magnecor KV85 leads. My plugs are Champion RN9PYP, also states 7415 on plugs. Plug gaps are .035". When I remove #4 plug and rest on engine, it sparks OK and strobe light shows it is firing, but in the cylinder strobe light shows erratic or no spark. Has my MSD voltage damaged this plug? What is a better plug solution, please?
Maybe too simple of a test, but have you tried swapping a couple of the plugs? Pull #2 and #4 plugs out, swap them, connect the wires back up, and see if you're still getting the misfire on #4. If it's now on #2, it's the plug. If it's still on #4, then it's farther upstream.