chore(docs): Release notes for 4.0

Author: wardpeet

docs/docs/reference/release-notes/v4.0/index.md

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+---
+date: "2021-10-21"
+version: "4.0.0"
+title: "v4 Release Notes"
+---
+
+Welcome to `[email protected]` release (October 2021 #1).
+
+This is the new major of Gatsby since [March 2021](https://www.npmjs.com/package/gatsby/v/3.0.0)!
+We’ve tried to make migration smooth. Please refer to the [migration guide](/docs/reference/release-notes/migrating-from-v3-to-v4/)
+and [let us know](https://github.com/gatsbyjs/gatsby/issues/new/choose) if you encounter any issues when migrating.
+
+Key highlights of this release:
+
+Major dependency updates:
+
+- [Node 14](#node-14)
+
+Also check out [notable bugfixes and improvements](#notable-bugfixes-and-improvements).
+
+**Bleeding Edge:** Want to try new features as soon as possible? Install `gatsby@next` and let us know
+if you have any [issues](https://github.com/gatsbyjs/gatsby/issues).
+
+[Previous release notes for 3.14](/docs/reference/release-notes/v3.14)
+
+[Full changelog](https://github.com/gatsbyjs/gatsby/compare/[email protected]@4.0.0)
+
+## Breaking Changes
+
+If you're looking for an overview of all breaking changes and how to migrate, please see the [migrating from v3 to v4 guide](/docs/reference/release-notes/migrating-from-v3-to-v4/).