There is really no need for a separate xfce4 tag, since virtually any question about xfce in Ubuntu will pertain to major version 4. Previous versions are not currently supported in Ubuntu. (They use GTK+ 1!) We might get a question about an earlier version on extremely rare occasion, but I don't think this merits a separate tag.
Still, I don't think xfce4 should be blacklisted. Other desktop environments have tags for their current versions, so xfce4 should exist for consistency. But (perhaps) unlike for some desktop environments, it shouldn't be a separate tag. People aren't using versions of Xfce prior to 4 these days. At least not often enough to merit a separate tag.