Cylinder number four misfire

Hi, I keep getting a misfire in cylinder four that brings on the check engine light. I see on the internet that a common fault is the plug seals on the valve cover, but with the plugs in, when I look down the plug holes with a torch they are all bone dry. When I take the plug out of cylinder four, the plug is all black and oily looking and the same when I look in the plug hole with plug out. Fitting a new plug solves things for a while. The car does use some oil, and cannot see any bad oil leaks. This car does not have the individual coil packs on the leads, it seems to be a separate block type? Can any one give me any help please? Thanks, Jim