The Enya #3 is a relatively hot plug. Its all I use in my two-stroke heli engines. I don't think it is suited for four strokes for two reasons:

It IS a "hot" plug, and it is a "short reach" plug. Compare it to the OS F (and now the YS clone of same) that is pretty much THE plug to use in four strokes; the F is longer than an Enya #3.

Again, this is my opinion formed from years of running all the mentioned items. I don't have any solid documentation to back it up.

Whatever the reason for the failures, I feel your pain; those puppies are very expensive.