Mine is high mileage, too. 180k. As I said earlier, it seems to clear up to an acceptable level as the operating temp rises, which got me thinking about it being a "cold" plug and maybe I would bump the heat range on #4 up a little. Hard to figure an engine that operates "normally" close to 210F could have a cold cylinder though.
BUT... I have also recently noticed the miss comes back if I shut down and re-start...like on a trip to the store. That kinda tells me the problem might reside in the closed loop "warm up" cycle of the computer. So now I'm wondering if at some point these high mileage cars might need to be re-flashed. Anybody tried that? ~FH