For years the string group I play in has been using Volume One of the Peters edition of the Mozart String Quintets. Perhaps strangely - though we've never thought to question it - the first volume includes quintets "4-8", though these are listed as "1-5" in the parts themselves. Recently we decided we wanted to look at the rest of the quintets, so I got hold of Volume Two, which contains quintets "1-3" and "9-10", numbered "6-10" in the printed parts. Volume One contains the best-known of the quintets (K.406, K.515, K.516, K. 593 and K. 614), while Volume Two has two early quintets (K. Anh. 179 and K. 46) which are only attributed to Mozart and may not have been written by him, the Quintet in B flat K. 174, and then two versions of the Clarinet Quintet K. 581 - one in the original scoring for clarinet and string quartet, and another in which the clarinet part is played by the first viola. The series is completed by the Horn Quintet K.407, with a first viola part that can accomodate the horn part.
So Volume Two is definitely something a mixed bag. I feel that there must be some more rational way of organizing the music, but there don't appear to be any other performing editions available. The Henle Urtext editions haven't got round to producing one yet. The Breitkopf edition does present the works in a more logical order, but there is apparently no commercially available set of parts to accompany it. Perhaps I'm wrong about this - would be grateful for information on the subject from anyone who knows.