4.0 introduces a newer version of Doctrine which includes a change to the namespacing of the AbstractMigration class.
If you have any migrations in your project that reference this class, the namespace reference will need to be updated.
Change from:
Doctrine\DBAL\Migrations\AbstractMigration
to:
Doctrine\Migrations\AbstractMigration
The method signatures for many methods within this class have also changed. Migrations extending this abstract will therefore also need to be updated, for example changing from:
public function up(Schema $schema)
to:
public function up(Schema $schema): void
A script is available to apply these changes to all migrations within a project.