chore: release 4.0.0 (#249)

* updated CHANGELOG.md [ci skip]

* updated package.json [ci skip]

* updated samples/package.json

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

Author: release-please[bot]

security-center/snippets/package.json

@@ -14,7 +14,7 @@
   "license": "Apache-2.0",
   "dependencies": {
     "@google-cloud/pubsub": "^2.0.0",
-    "@google-cloud/security-center": "^3.1.0"
+    "@google-cloud/security-center": "^4.0.0"
   },
   "devDependencies": {
     "chai": "^4.2.0",