Despite what is written above, and drawing on my own experiences and after in depth consultatuion with Denso, I have found that the Denso Iridium 0.4mm plug lasts as long as the 0.7mm plug and have experienced no issues or failures within 54,000 miles on either plug on the hundreds of sets that I have sold or fitted.
I would replace any Denso Iridium plug for free that fails within that period without question on any N/A engine.
Alex