Upgrading from 3.x to 4.x should be trivial. The public API is identical; other changes include:
PHP 8.1 is the minimum required version.
The Definition::property() method now injects the property
value before object construction, not after, and will work on any
property, not just public properties. To get the old behavior of injecting
a public property after construction, use the Definition::modify() method
to set the property directly on the object.