There is oil on one spark plug, cylinder number four to be specific, all of the spark plugs are bone dry. What is most likely causing Oil to be on that spark plug?
My car has 200000 miles.
There can only be two ways the oil is getting on the spark plug.
The piston oil control rings are stuck or broken and letting excess oil into the cylinder.
The intake valves seals for the cylinder in question are leaking oil into the cylinder due to bad seals or excessive oil above the valves from blocked oil drain back holes on the cylinder head is clogged.
These are the possible reasons for the oil getting into the cylinder. A cylinder bore scope can detect to see if the oil is coming from the intake valve possibly, but other than that, removing the valve cover to see if the oil is draining back properly and inspecting the valve seals is only way to see the valve problem.
If no problems are seen there then the problem will be the piston rings.
I suggest having your oil on spark plug issue inspected by a qualified mechanic.