1999 4.0 liter plug #4 foul twice now

[Rolyat]

I have had check engine come on twice in the last 4000 miles, OBD code P0304. This was cleared with a new spark plug in that cylinder. The first time I tried things other than the plug, because my plugs were relatively new Autolite double platinum, which are supposed to last a long time. I paid $90 for one new plug at a shop. The second time, I cunningly did it myself for the cost of a plug, successfully.

This is a 1999 4.0 liter Ranger EXT 4WD.

What could be the cause of this? Plug looks brown, gap is there with a bit of extra metal on the electrode, but not a lot.

Thanks, Roly

[12Ounce]

High on my list would be coil (does your engine have a coil on each plug?) ... or coil pack/ignition cable. After ignition items, I would investigate the injectors ... by swapping a couple of injectors you could see if the problem changed locations.