The document compares the directory structures and MVC implementation between CodeIgniter 3 and CodeIgniter 4. Some key differences include CodeIgniter 4 using namespaces for classes, updating the directory structure of application and system folders, and implementing an ORM for models to work with databases rather than using the query builder as in CodeIgniter 3. The index.php file is also update
![Codeigniter4の比較と検証](https://cdn-ak-scissors.b.st-hatena.com/image/square/1e1899d18c60f51eb91cc3dfd080dcf7d8daa5e5/height=288;version=1;width=512/https%3A%2F%2Fcdn.slidesharecdn.com%2Fss_thumbnails%2Fcodeigniter4-160903141747-thumbnail.jpg%3Fwidth%3D640%26height%3D640%26fit%3Dbounds)