As of recently, there are two individual characters, Koyanskaya of Light and Koyanskaya of Darkness. The latter has Beast IV:L's characteristics, the former doesn't. Using Pixiv tags as an example, keep in mind artists/users already differentiate them by name (either by Koyanskaya of Light and Koyanskaya of Darkness or simply by Koyanskaya and Koyanskaya of Darkness).
IMO there are four possible courses of action to differentiate them on Danbooru, like we do with other characters with "reused design":
1 - Rename Koyanskaya to Koyanskaya of Light and Beast IV:L to Koyanskaya of Darkness;
Tamamo could remain implied, functioning as an umbrella for both plus the other Tamamos or have the implication removed, since similar to how Gray and other saberfaces don't count as Artoria Pendragon, Koyanskaya isn't really a Tamamo.
2 - Create Koyanskaya of Light and rename Beast IV:L to Koyanskaya of Darkness;
Tamamo could remain implied ... or have the implication removed... Koyanskaya could be kept as an implication, catchall tag. It'd make things easier but be somewhat meaningless because anyone can search for "koyanskaya_of_light_(fate) koyanskaya_of_darkness_(fate)".
3 - Keep the tags, using Beast IV:L solely to tag art of the Beast itself and Koyanskaya of Darkness' ascension resembling her and everything else under Koyanskaya.
The easiest way, but like Kiara, Tiamat, Goetia and Kama's Beast forms, I think Beast IV:L shouldn't have a character tag.
4 - Alias Beast IV:L to Koyanskaya.
I think it's the worst, simply bad. People could search for specific tags to find Koyanskaya of Darkness' art, but it would be a problem for those of us who have a limit of two tags per search.
What are your opinions and ideas regarding what should be done with these characters?