[chore] Release 4.0.0 (#415)

Author: hiranya911

.github/workflows/release.yml

@@ -129,7 +129,7 @@ jobs:
     - name: Post to Twitter
       if: success() &&
         contains(github.event.pull_request.labels.*.name, 'release:tweet')
-      uses: ethomson/send-tweet-action@288f9339e0412e3038dce350e0da5ecdf12133a6
+      uses: firebase/firebase-admin-node/.github/actions/send-tweet@master
       with:
         status: >
           ${{ steps.preflight.outputs.version }} of @Firebase Admin Python SDK is avaialble.

firebase_admin/__about__.py

@@ -14,7 +14,7 @@
 
 """About information (version, etc) for Firebase Admin SDK."""
 
-__version__ = '3.2.1'
+__version__ = '4.0.0'
 __title__ = 'firebase_admin'
 __author__ = 'Firebase'
 __license__ = 'Apache License 2.0'