Upgrading from 3.x to 4.x
League\Period 4.0 is a new major version that comes with backward compatibility breaks.
This guide will help you migrate from a 3.x version to 4.0. It will only explain backward compatibility breaks, it will not present the new features (read the documentation for that).
If you are using composer then you should update the require section of your
composer require league/period:^4.0
This will edit (or create) your
PHP version requirement
League\Period 4.0 requires a PHP version greater or equal than 7.1.0 (was previously 5.5.9).