I've had engines suck the coil out of the plug a few times. I've even pulled a blown plug to find the element hanging there, pulled well out of the plug body. I've had plug elements "mashed" up inside the plug body, and elements just missing. I've never had an engine get badly damaged by a plug element, but I have found scratches on the piston/liner that I can't account for any other way, and I've found bits of metal on the exhaust port that weren't there before (a bit of work with a file and emory cloth fixed that one). But I've never had an engine loose real performance from a plug element. But I've seen it happen to others. I think it's pretty rare though.

4 gallons, yep, that's a darn good plug lifespan. Btw, I happen to use K&B plugs in most of my engines. I order them direct from K&B over the phone, usually 12 at a time, which last for quite a while. (keep in mind, I fly a LOT of combat, I currently have about 15 engines just for combat. Plus sport engines).