Powerstore 3 should continue to function when you install powerstore 4 using the same database.
However, it will import all of the data on installation, and won't be in sych after editing or taking orders in your powerstore 3 site. You would probably want to install into a different directory using the same database, then do your testing and design updates to get ready to go live. When you are ready to go live, then delete the /Connections/ and the /webassist/ folders as well as any tables with names that begin with "ps4".
Then you can install again and it will create a fresh copy of the database from the ps3 version.
We have not determined the final feature-set of the next version of powerstore. We appreciate all suggestions and will consider that option as we investigate potential additions.
I do appreciate the suggestion that we might want to time this product specifically for after the holidays, but on the flip side a lot of people are rushing right now to get sites up and the leaves haven't even turned yet.