Add v4.x guide to root README.md

Author: filipdjokic

SUMMARY.md

@@ -31,6 +31,7 @@
     * [Upgrade to v2.x (with Ubuntu 24.04 LTS upgrade)](docs/upgrades/upgrade-guides/v2.x-upgrade.md)
     * [Upgrade to v3.x](docs/upgrades/upgrade-guides/v3.x-upgrade.md)
     * [Upgrade to v3.1.x](docs/upgrades/upgrade-guides/v3.1.x-upgrade.md)
+    * [Upgrade to v4.x](docs/upgrades/upgrade-guides/v4.x-upgrade.md)
   * [Creating a software upgrade proposal](docs/upgrades/propose-software-upgrade.md)
   * [Upgrade Process](docs/upgrades/upgrade-process.md)