[chiphead]

How concerned should we be for .004"? If you don't want to tear it down, you should take it easy on the engine. No setting the tune on kill and no forced induction/nitrous. You'd definitely want to run it rich and keep the timing down. Extended high speed interstate runs should be done with caution. The high levels of silicon are great for dimensional stability, but they don't conduct heat well to the second ring or the skirt. So more heat goes into the top ring. If the ring material tends to expand with heat, it'll butt.

Is this 1/16" moly cast ring, a ductile iron ring, a AP steel, what is it?

I've seen threads where the top of a KB piston got torn off. In those cases, I believe the builder gapped the top ring like a N/A moly ring on a cast piston, like .016". That would cause me night sweats for sure.