When I checked this once after engine shutting down in high speed run, the starboard side plugs looked "normal" like they'd been dipped in dirty motor oil with a little brn/blk surface deposits which I could wipe off. The port side looked like they'd been rinsed in clean fuel-oil mix. I thought that would point to the port side switchbox, quitting due to a heat breakdown problem. Also, at least once, after idling/flushing engine, #4 plug also looked clean while all the others looked like they'd been firing.

Trollercoaster