I don't think the series 4 were galvanized, the series 3 was certainly not galvanized. However, the series 4 was the first spider to have proper rust protection.
I have no idea what they actually did to prevent rust. But the series 4 is well know for it as being the first model that didn't rust badly. And even today you can find rust free Series 4 Spiders. My 1990 Spider is - which I sadly don't own anymore - is also still free of rust on the bodywork.