91 octane or higher. I use 93 or 91 for my '02. check your manual, and it should also say on the inside of your gas lid cover.
I don't have an '04, but my '06 takes Premium. It says so on the inside of my gas lid. I use 93 at Shell (V-Power). I figure if that's what Ferrari uses in their F1 cars, then it's good enough for mine =D
fueleconomy.gov shows all the miata's from that year require premium, so 91 octane (R+M)/2 or I think it's like 96 RON if you happen to be where they use that system.
Looking for a Used MX-5 Miata in your area?
CarGurus has 4,223 nationwide MX-5 Miata listings starting at $2,000.
Search Mazda MX-5 Miata Questions
Mazda MX-5 Miata Experts
Related Models For Sale
Used Cars For Sale