Yes, the center electrode did not burn off 'cause it was never really there. I feel a little silly, and probably now no one will believe me that these spark plugs were not working very well after 4000 miles. There is always the possibility that my engine runs rich, which was the root cause of the plugs fouling. In any case, new plugs work better.

And now I much more fully appreciate the comment that the gap can't really be changed. I am under the general impression that late model vehicles have higher energy ignition systems which allows for wider plug gaps (which gives larger arcs, better burn, etc.) So does this possibly explain why these may not be suitable for certain generations of ignition systems/engines? That is, the gap is just a little too big off the shelf?