I've been thinking about this for a while now. There are a lot of posts with 4 (or more) copies of her and it's only going to increase in the future. I'll gladly populate the tag myself, I just want the approval to do so.
Before it's said, yes I know Flandre_Scarlet clone or Flandre_Scarlet Multiple_Persona would cover that one tag. My reason behind this is, even if those tags cover it, it's still an active spellcard. Without the danmaku in effect. Since it's a specific ability that she does, it should probably be tagged as so. There are other spellcard tags, such as Master Spark and Spear the Gungnir follow in the same category.
So, to sum this up, even though it's called "Four of a Kind", the tag can be used when there are 3 or more clones in a picture. If there are 2, that can be covered with Flandre_Scarlet clone or Flandre_Scarlet Multiple_Persona.
(So far, there are 5 pages worth, not including the posts that haven't been tagged correctly yet. I'll have to find those myself.)
Updated by BlueFox