Where is the 4th spark plug?
Look on top of the engine and you will see 4 ignition coils. Directly under each coil in the tubes through the valve cover is where the spark plugs are placed. Remove all 4 coils and look down the tubes with a flashlight and you will see all the spark plugs. If you are unable to see the spark plug but you see a lot of black stuff, then the valve cover is leaking at the top of the tube causing oil to drip down on top of the spark plug. You would need to use a pump to suck out the oil before removing the spark plug so the exhaust system does not get coated with oil as the oil will go into the cylinder. If you need further assistance with your spark plugs being visible, then seek out a professional, such as one from Your Mechanic, to help you.