4.0.x: Pin protobuf < 5 (#1598)

For 4.0.x pin protobuf < 5 as in #1421.

#### What does this implement or fix?

#### Any other comments?

#### Checklist

<details>
  <summary>
   Checklist for code changes...
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- [ ] Have you updated the relevant docstrings, documentation and
copyright notice?
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features](../docs/mkdocs/docs/technical/contributing.md)?
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setup.cfg

@@ -39,7 +39,7 @@ install_requires =
     attrs
     dataclasses ; python_version < '3.7'
     grpcio-tools
-    protobuf >=3.5.0.post1 # Per https://github.com/grpc/grpc/blob/v1.45.3/requirements.txt
+    protobuf >=3.5.0.post1, < 5 # Per https://github.com/grpc/grpc/blob/v1.45.3/requirements.txt
     xxhash
     six
     msgpack