It's impossible for us to know. "4" is passed to the article_details.php script, and that script is apparently doing something with "4" to show you an article. You'd need to look though that script and see what it does with the parameter "article_id"
The contents of the articles are either stored in a database or pulled from other external files. That's why (a) they use a single .php script to fetch the article content and display it, and (b) changing the value for the 'id' parameter in the query string selects the article to be displayed. If the articles were not in a database, the site would be much more likely to use conventional HTML files and you would be able to locate them individually. You would need to examine the source code for 'article_details.php' in order to know exactly where the data comes from, and to do that you would need direct access to the files on the server.