Was...missing #4. Now Fouled spark plug

Hi Folks, I posted a few days ago about missing in #4 cylinder. As was advised I had a badly fouled plug in that cylinder. I cleaned the bad plug and the engine runs like a dream. Question. Why is my engine fouling #4 plug and what can I do to correct this problem? Thanks, Enrqiue

It could have been nothing more than a piece of carbon broke loose and lodged between the electrodes of the spark plug. That was pretty common stuff before about 1990 but may never happen to you again.