Hello,
Yes, this article is for X-Cart 4.
For placing "Add to cart" buttons on an external site for X-Cart 5 please refer to this article:
http://kb.x-cart.com/display/XDD/Adding+products+to+cart+via+URL
As for the checkout button - it is nothing more than just a link to the http://<your-domain>/<x-cart-5-directory>/cart.php?target=checkout page.