I use the 4.x eCommerce engine on several production environments and the features you've mentioned are pretty interesting. I already had to rewrite the CC gateway to work with my commerce server for processing, but its pretty proprietary. If you have any questions about doing that I may have answers.
My biggest problems with the older/current version is its inconsistent templating implementation and its use of db api that can break down if you try to integrate any of the stores api on other pages in the site.
-Jacob
