Pretty straightforward stuff. We generally use the
-[#]e tag suffix to denote different editions (presumably because that's how those editions of those games are commonly abbreviated) - and the tags for the previous three editions of Warhammer Fantasy Roleplay follow this trend: wfrp-1e, wfrp-2e, and wfrp-3e. (There's also a general wfrp tag, but that's not relevant here.)
The wfrp-4 tag was created a little over a month ago; I'm guessing the person who created it wasn't aware of the convention for edition tags.