i believe FC 4 lug bolt pattern is 4x108, thats why everyone goes 5 lug with FC. id make it harder to find wheels than with 4x100. if you want to upgrade bolt patterns to FC 5x114.3 id make more sense, more aftermarket wheels backup. ive heard of a couple people doing it with easy, you need a custom lower balljoint i think. im not sure of the details, someone around here might know.
search back in some previous posts, i think someone asked it on this site once.
i've seen one fella working on an fc 5lug hub conversion. from what i saw of his work, you had to chop the upper control arm mount, and refab a whole new one for the miata's upper control arm. iirc, he only got as far as the front. i haven't seen what it would take to do rears. it looks like a lot of work.
if you just want a 4x114.3 pattern, get the adapters, like these