Documentations

Email Documentation CodeIgniter 3.x <http://codeigniter.com/userguide3/libraries/email.html>_
:doc:Email Documentation CodeIgniter 4.x </libraries/email>

What has been changed

Only small things like the method names and the loading of the library have changed.
The behavior when using the SMTP protocol has been slightly changed. You may not be able to communicate with your SMTP server properly if you use the CI3 settings. See :ref:email-ssl-tls-for-smtp and :ref:email-preferences.

Within your class change the $this->load->library('email'); to $email = service('email');.
From that on you have to replace every line starting with $this->email to $email.
The methods in the Email class are named slightly different. All methods, except for send(), attach(), printDebugger() and clear() have a set as prefix followed by the previous method name. bcc() is now setBcc() and so on.
The config attributes in app/Config/Email.php have changed. You should have a look at the :ref:setting-email-preferences to have a list of the new attributes.