I'm not sure this is the right place, but I have a question about the story categories. Each of the Buffy series are listed seperately, plus a different category labelled "Alternate Universe". What sort of story lands in the AU one and which go under the appropriate series?

The story in question I'm trying to submit is set in series four, but only takes into account some of the canon from the beginning of the series onwards - i.e, keeping Riley and the Initiative, ignoring pretty much everything else. So, Series 4 or AU?

I'd go with S4... the AU section is more for things that don't start in canon. But it's up to you, really...they are more a guideline than anything.