i cant help you on the 3 plug looking different but ive always heard for the roadstar the rear cylinder runs richer, but that should only affect 2 not three unless some air leak some where just for that part of the cylinder not a mechanic but it my best knowledge
Guess you misread... 1 plug is dirty.
the other 3 are burning nice and clean.
And as far as the copper wire, the resistance in the cap is supposed to be 10k ohms. The one in question is 9.73k ohms.
The resistance with copper wire in the cap is 3 ohms (not k ohms)
Been there, Done that.
Checked for spark too. It sparks, nice fat blue one.
What i don't know is if it's a consistent spark.
Perhaps an occasional misfire? Don't know.
Hoping some of the experts may know the cause or have the solution.