Upgrade Output Class
Documentations
What has been changed
- The Output class has been changed to the Response class. 
- The methods have been renamed. 
Upgrade Guide
- The methods in the HTTP Response class are named slightly different. The most important change in the naming is the switch from underscored method names to camelCase. The method - set_content_type()from version 3 is now named- setContentType()and so on.
- In the most cases you have to change - $this->outputto- $this->responsefollowed by the method. You can find all methods in HTTP Responses.
Code Example
CodeIgniter Version 3.x
<?php
$this->output->set_status_header(404);
// ...
$this->output
    ->set_content_type('application/json')
    ->set_output(json_encode(array('foo' => 'bar')));
CodeIgniter Version 4.x
<?php
$this->response->setStatusCode(404);
// ...
return $this->response->setJSON(['foo' => 'bar']);