There are no "fourth parties" just as there are no "second parties" lmao
Either you're first party which means you belong to that company (or at least co-owned or something), or you're a third party which means you don't belong to that company, simple as that.
As they're not owned by Nintendo, the likes of Spongebob, Goku, Optimus Prime, Batman, etc are just as "third party" as Sonic, Snake, Cloud, etc are...it's just that they're not video game characters and thus not eligible for Smash, that's the real distinction.
Has Sega Hard Girls ever even gotten a western localization? I don't think that would ever be referenced in Smash either way.