I tried using '4.1' and '4.2' in the title to indicate where my shorts fitted into the series, then changed my mind again. Now the numbering is only in the description, but displayed in bold face and right up front.
Why? Because I noticed people were buying books 1-3, then 4.1 and 4.2, then book 5, 6, etc. Sales of book 4 - the real novel - actually dropped off for several days.
However, book 7 is a serialised novel so I'm happy to publish it as 7.1, 7.2, etc.