[WIP][翻訳][Part4]42ad0ad4c5まで更新

Part4を英語版の https://github.com/dsyph3r/symblog-docs/commit/15d15cee5ccf72eca1708e3d5d8e0ef9ffdeb207#diff-827edf1bcdceb9b79f1e6db4e030bec7 まで更新。

```
git diff 1a83e511ac9fbda13d8eafa97c980614a5d9d397..HEAD docs/extending-the-model-blog-comments.rst
```