To correct this, you'll need to update the Store URL in your Shift4Shop settings.
You may do this by using the following steps. From your Online Store Manager, go to:
Settings > General > Store Settings.
Click on the Store tab at the top of the section.
Change the "Store URL" to reflect your store URL i.e. http://www.yourwebsite.com
Once this is done, http://www.yourwebsite.com/sitemap.xml will show the correct addresses for the URLs.