I had a seat of the pants vibration in my plane and what I perceived to be a bit of a rough running engine. My mag checks were all good and my engine monitor did not show anything wrong. I decided to pull and clean my plugs and I noticed that the bottom plug on cyl 4 is different than all the others. I have a O-360 engine and all my plugs were 40E except for that one which was a 38YB. I called my mechanic and he hd no clue. Ive been flying with the plane like this for at least the last 50 hours and only noticed the issue in the last 10 hours or so. Any ideas?