As far as I know, ModMii was never made available to Mac users. There's even an April fool's day gag going on today about a new version of ModMii available for Mac and Linux.
You should be fine on System Menu 4.0. All you'd need to update is cIOS249. cIOS250 is only needed if you have problematic Homebrew that is experiencing issues with 249.
I don't remember which SM added IOS58. I know that a lot of current Homebrew runs off of that. If it was SM 4.1 that added IOS58 then you might want to find a friend with a Windows laptop/PC that you can use ModMii on.
You should post a SysCheck here so we can see what might need to updated. Maybe you don't need much, and can do it without ModMii.
You should be fine on System Menu 4.0. All you'd need to update is cIOS249. cIOS250 is only needed if you have problematic Homebrew that is experiencing issues with 249.
I don't remember which SM added IOS58. I know that a lot of current Homebrew runs off of that. If it was SM 4.1 that added IOS58 then you might want to find a friend with a Windows laptop/PC that you can use ModMii on.
You should post a SysCheck here so we can see what might need to updated. Maybe you don't need much, and can do it without ModMii.