ID3v2.3 is the “de facto” tagging standard for MP3, like it or not.
ID3v2.4 is a still-born informal standard, IMHO. It never caught on and didn’t even make it to the original “id3lib”. (foobar2000 being the most popular application to force it’s usage on MP3 content — it will read older, but will only write to 2.4.)
I’d recommend to use ID3v2.3 exclusively for interoperability, unless you have a unique need or are addicted to foobar2000’s tagging interface.
NOTE: In my experience, you will experience duplicate and bad tags in your MP3 files if you start editing/writing the same MP3 files with 2.3 and 2.4 centric applications… many tend not to behave very nicely.
But as I said before, both “standards” suck… 8)
But as Ron said… Firefly’s libid3tag should work with many ID3v2.4 tagged files, though.