CodeIgniter 4 is a rewrite of the framework, and is not backwards compatible. It is more appropriate to think of converting your app, rather than upgrading it. Once you have done that, upgrading from one version of CodeIgniter 4 to the next will be straightforward.

The "lean, mean and simple" philosophy has been retained, but the implementation has a lot of differences, compared to CodeIgniter 3.

There is no 12-step checklist for upgrading. Instead, start with a copy of CodeIgniter 4 in a new project folder, however you wish to install and use it, and then convert and integrate your app components. We'll try to point out the most important considerations here.

Not all of the CI3 libraries have been ported or rewritten for CI4! See the threads in the CodeIgniter 4 Roadmap subforum for an uptodate list!

Do read the user guide before embarking on a project conversion!

Downloads

• CI4 is still available as a ready-to-run zip or tarball, which includes the user guide (though in the docs subfolder
• It can also be installed using Composer

Namespaces

• CI4 is built for PHP7.1+, and everything in the framework is namespaced, even the helpers

Application Structure

• The framework still has application and system folders, with the same interpretation as before
• The framework now provides for a public folder, intended as the document root for your app
• There is also a writable folder, to hold cache data, logs, and session data
• The application folder looks very similar to that for CI3, with some name changes, and some subfolders moved to the writable folder
• There is no longer a nested application/core folder, as we have a different mechanism for extending framework components (see below)

Class loading

• There is no longer a CodeIgniter "superobject", with framework component references magically injected as properties of your controller
• Classes are instantiated where needed, and components are managed by Services
• The class loader automatically handles PSR4 style class locating, within the App (application) and CodeIgniter (i.e. system) top level namespaces; with composer autoloading support, and even using educated guessing to find your models and libraries if they are in the right folder even though not namespaced
• You can configure the class loading to support whatever application structure you are most comfortable with, including the "HMVC" style

Controllers

• Controllers extend CodeIgniterController instead of CI_Controller
• They don't use a constructor any more (to invoke CI "magic") unless that is part of a base controller you make
• CI provides Request and Response objects for you to work with - more powerful than the CI3-way
• If you want a base controller (MY_Controller in CI3), make it where you like, e.g. BaseController extends Controller, and then have your controllers extend it

Models

• Models extend CodeIgniterModel instead of CI_Model
• The CI4 model has much more functionality, including automatic database connection, basic CRUD, in-model validation, and automatic pagination
• CI4 also has the Entity class you can build on, for richer data mapping to your database tables
• Instead of CI3's $this->load->model(x);, you would now use $this->x = new X();, following namespaced conventions for your component

Views

• Your views look much like before, but they are invoked differently ... instead of CI3's $this->load->view(x); you can use echo view(x);
• CI4 supports view "cells", to build your response in pieces
• The template parser is still there, but substantially enhanced

Libraries

• Your app classes can still go inside application\Libraries, but they don't have to
• Instead of CI3's $this->load->library(x); you can now use $this->x = new X();, following namespaced conventions for your component

Helpers

• Helpers are pretty much the same as before, though some have been simplified

Extending the framework

• You don't need a core folder to hold MY_... framework component extensions or replacements
• You don't need MY_x classes inside your libraries folder to extend or replace CI4 pieces
• Make any such classes where you like, and add appropriate service methods in application/Config/Services.php to load your components instead of the default ones