Quad electrode: I measure the distance between the electrode and the tip of the prong. It is always more than 0.042". In theory, the quad sounds great, in reality, spark can only jump to one prong. IMHO, 0.042" is too much of a gap.

I learned the Iridium lesson from my 2007 Honda Odyssey van, which came from factory with NGK Iridium. At 65K, it looks like new, gap was still within spec. I was sold.

Then I started doing research for my E39, put NGK Iridium (gap 0.028") a few years ago, engine smooth like silk, no hiccups.

Ditto for my X5, smooth, fuel mileage is better than the quad thingy that I threw away.

The best bang for the bucks is, IMHO, NGK Iridium $7/each. Better than BMW dealer pricing!

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT