[Translations] Japanese ja_JP - Chapter 4

[nwiizo]

📝 Description

Continues work on #61
/kind documentation

Dear @salaboy and the platforms-on-k8s team,

Thank you for merging the previous chapters' translations. I appreciate your ongoing support and feedback. 🙏

This PR adds the Japanese translation for Chapter 4 of the "Platforms on Top of Kubernetes" book. 📚

🔄 Changes

• ➕ Added Chapter 4 content to README-ja.md
• 🔍 Ensured consistency in formatting and structure with the original English version and other translations
• 🔄 Updated the table of contents in README-ja.md to reflect the addition of Chapter 4

📌 Notes

• 🎯 As with previous translations, I've focused on maintaining accuracy while ensuring the content is accessible to Japanese readers.
• 🌏 This continued effort aims to further benefit the Japanese-speaking Kubernetes and cloud-native community.
• 💬 I welcome any feedback or suggestions for improvement. Please don't hesitate to point out any areas that need refinement.

Thank you for the opportunity to contribute. I look forward to your review. ✨

[salaboy]

Thanks a lot @nwiizo