Help with a misfire

So I recently acquired a 2000 nissan frontier (3.3L) from my grandfather. Went to a local car parts store to see if any errors ran on the vehicle, and I got a code stating that the knock sensor was bad. At this point there were no misfires, and the engine didn't run efficiently. So I figured the plugs probably haven't ever been changed, and went ahead and changed them, along with new wires. After that, the misfires started. The only happened underload uphill and at speeds greater than 55 mph. No other times.

Took all the plugs out, and found spark plug #4 had oil on the threads, as well as a little bit on the electrode. Went to run codes and the only codes I got was that bad knock sensor. I thought hmm, maybe it's the valve cover gasket leaking oil into the plug. Just got it apart to replace the knock sensor, and boom no signs of leaks anywhere on it. So my question is, is it what I fear? Is it the Rings on the cylinder? I'm running out of ideas here. This is my first endeavor into anything this serious when it comes to D-I-Y mechanics, so anything would be greatly appreciated. Thank you. ::smile::