this i my cousin's car but i figured maybe i can get some good answer's for him in here

my 03 spec for some reason is eating the plug in cylinder number four it did it a few months ago i changed the plugs and it was fine. yesterday i took the car started running on 3 cylinders. i take the same number plug out again and the ground strap was pushed down on the electrode which was chipped, and the base of the plug was super pitted any ideals? detonation something in the combustion chamber i am stumped. please help o will post pics of the plug later