There is not documentation available that is specific to V4 at this time, but the concepts in the V5 documentation are still the same.
The process of creating a store using eCart from v4 to v5 is not changed:
1) Create the product catalog
2) Create the eCart object
3) add the Add to cart buttons to your product pages.
4) Create a page to display the cart contents using the Cart Display manager
5) Create the checkout pages using the Checkout wizard.
the main differences between V4 and V5 will be in the way the interfaces look an some of the options. the core concepts are still the same.
the support files you downloaded should still work also.
If there is a particular step you are unsure of, ask here.