diff --git a/installation/upgrade-notes.md b/installation/upgrade-notes.md
index 68d21f8cb..fdcd50f98 100644
--- a/installation/upgrade-notes.md
+++ b/installation/upgrade-notes.md
@@ -9,6 +9,18 @@ Note: release notes will be prepared in advance of a Git tag for a release so an
 The tag drives the overall binary release process so release binaries (containers/packages) will appear after a tag and its associated release note.
 This allows users to expect the new release binary to appear and allow/deny/update it as appropriate in their infrastructure.
 
+## Fluent Bit v4.0+
+
+By default, configuration files use YAML syntax and the `.yaml` file extension. 
+
+You may need to update your custom deployments to either use YAML configuration files
+or switch back to using legacy configuration in the arguments to the Fluent Bit binary.
+
+In Linux environments, you must override the systemd unit to specify `-c <legacy config file>`.
+
+Similarly, in Windows and macOS container environments, the default launch command uses YAML configuration files
+by default.
+
 ## Fluent Bit v1.9.9
 
 The `td-agent-bit` package is no longer provided after this release.