Pull the plug out of that cylinder and inspect it.
If you don't know what to look for, go to any local parts store and find a Haynes repair manual. Look in the back for an in-color page with pictures of bad-burning sparkplugs. That should help you determine what's fouling that plug.
Most likely the rings in that particular cylinder have worn down quicker than the rest. This could be due to many reasons. Something as simple as a few grains of sand could've gotten in there and worn down the walls or rings. They don't have to be worn a lot, just enough to allow oil to blow by which will slowly foul that sparkplug.
It wouldn't hurt to try a spark plug spacer/adaptor. You screw the plug into the spacer/adaptor and then screw the adapter into the engine where the plug goes. It backs the plug up a little ways from its usual seating so whatever chemical keeps fouling it can't get to it as easily.