I couldn't find Bosch 4s for the Z; they're the best plug I've used in my wife's Corolla and my Nissan Hardbody. Both the Corolla and truck had a slight miss w/ copper and Bosch 2s, the Bosch 4s smoothed it out.
I guess it's a matter of preference but I do notice a difference in types of plugs; by far, iridium are best and preferred in my book. I've never 'measured' the performance difference; I don't have to or really desire to as it's that noticeable to me. Just my two cents, that's all.