Engine misfire causes

[spookybathtub]

All of a sudden, today my engine started running rough, and I got a P0304 trouble code (cylinder 4 misfire). As I work my way through the repair guide, I noticed that spark plug 4 has metal bits caked on the threads. Is this bad?

About two years ago, I had that same plug blow out, and a mechanic fixed it with a helicoil. I worry now that maybe the helicoil is disintegrating, and soon may blow out like last time. I wonder why this would happen twice in the same cylinder. I'd like to fix this myself if possible, but I still have a lot to learn about engines.

Here is a photo of plug 4 (top) and plug 3 (bottom). Full-res photo: https://dl.dropboxusercontent.com/u/...9/IMG_0237.jpg