Can confirm the issue. Until its resolved you could temporarily mark your EventSubscriber lazy:

diff --git a/composer.json b/composer.json
index 49746b5..4dd4ca4 100644
--- a/composer.json
+++ b/composer.json
@@ -15,6 +15,7 @@
         "symfony/monolog-bundle": "^3.1",
         "symfony/orm-pack": "*",
         "symfony/process": "^4.1",
+        "symfony/proxy-manager-bridge": "^4.1",
         "symfony/security-bundle": "^4.1",
         "symfony/serializer-pack": "*",
         "symfony/swiftmailer-bundle": "^3.1",
diff --git a/config/services.yaml b/config/services.yaml
index f98bd4e..2075d09 100644
--- a/config/services.yaml
+++ b/config/services.yaml
@@ -33,3 +33,6 @@ services:
 
     # add more service definitions when explicit configuration is needed
     # please note that last definitions always *replace* previous ones
+
+    App\Event\EntitySubscriber:
+        lazy: true

edit: ah it actually seems like just enabling lazy services in general (adding the proxy manager bridge) is already enough.

Thanks for the reproducer.

The cause of the issue is a circular reference when proxy-manager-bridge is not installed: on 3.3, this is detected at runtime and leads to [Symfony\Component\DependencyInjection\Exception\ServiceCircularReferenceException] Circular reference detected for service "doctrine.orm.default_entity_manager", path: "doctrine.orm.default_entity_manager -> doctrine.dbal.default_connection". (thus, not sure what you meant by "It worked in symfony 3.3": it doesn't "work" - but it fails properly yes :))

On 3.4 & up, this circular reference is undetected and turns to an infinite loop.

Oh, I see. In my 3.3 project I have an installed doctrine/doctrine-migrations-bundle. And It is somehow solve my problem just like symfony/proxy-manager-bridge do it for 4.1 version.

I have try on new install sf3.3. If there is migrations-bundle - no error, otherwise Circular reference detected.

Anyway installing symfony/proxy-manager-bridge solves problem in 4.1, thanks.

that's because doctrine/doctrine-migrations-bundle requires the ocramius/proxy-manager library so this one got installed, and your 3.3 project was probably using symfony/symfony rather than component packages, so the code of the bridge was also available. So this leads to a situation where you have the proxy manager bridge able to handle lazy services.