There is currently a Street_Fighter_IV tag, and 27 posts with it. In other situations, the originating series is used as a tag. AFAIK, SFIV is the only exception where a specific title from the series is being used as a tag.
The issues I have with this tag is a) limited usage, and b) inconsistency.
A) This tag will only be used for pictures related to Street Fighter that would already have the generic SF tag. The only pictures that would get SFIV added as well are ones that include a new character (Seth, Viper, or Abel), are official art used in SFIV, or feature SFIV-specific alternate outfits.
Perhaps these would be useful to some, but they seem like corner cases to me, especially since searching for the new characters is simple enough without the need for an SFIV tag.
B) No other games have a tag of their own like this (or else I haven't been paying attention). It would be a lot of work to apply new tags like this.
If this is consistently applied, every post with Kasugano Sakura in it should have a Street Fighter Zero 2 (Alpha 2) tag. Ryu and Ken would go under the Street Fighter 1 tag, but Chun-Li and Cammy would get a Street Fighter 2 tag. If they were wearing their alpha outfits, then Chun Li would also get a Street Fighter Zero tag, and Cammy would get a Street Fighter Zero 3 tag. Actually, Cammy would get the X-Men vs. Street Fighter tag, I think, technically.
We'd need tags for every single SF game so that we could tag the official art appropriately, and label each post that contains a new character or different outfit. If we were going to be anal about it, anyway.
In the end, however, it just goes back to effort:usefulness. In my opinion, it represents a lot of effort, and almost no usefulness. I would be interested to hear other opinions.
Updated by NCX