Engine light on 96 Mercury mystic/ cylinder 4 not firing?

Hello My 96 Mercury Mystic has the engine light on advance auto codes says cylinder 4 is not firing I replaced the distributer spark plugs distributer cap light goes off then comes back on after 2 weeks when light comes back on spark plugs are covered in oil. Replaced valve cover gasket 3 times still leaks oil into spark plug chambers and check engine light will come back on