Fetch external content via Ajax for the tabs by setting an href value in the tab links. While the Ajax request is waiting for a response, the tab label changes to say "Loading...", then returns to the normal label once loaded.
Tabs 3 and 4 demonstrate slow-loading and broken AJAX tabs, and how to handle serverside errors in those cases. Note: These two require a webserver to interpret PHP. They won't work from the filesystem.