diff --git a/README.md b/README.md
index 0ee137c..9d904cd 100644
--- a/README.md
+++ b/README.md
@@ -46,3 +46,7 @@ Be aware of S3 transfer costs if your runners are not in the same AWS region as
 * `RUNS_ON_RUNNER_NAME`: when running on RunsOn, where this environment variable is non-empty, existing AWS credentials from the environment will be discarded. If you want to preserve existing environment variables, set this to the empty string `""`.
 * `RUNS_ON_S3_FORCE_PATH_STYLE` or `AWS_S3_FORCE_PATH_STYLE`: if one of those environment variables equals the string `"true"`, then the S3 client will be configured to force the path style.
 
+
+## Action pinning
+
+Contrary to the upstream action, `v4` is a branch. As such, any tag pushed to this runs-on/cache repository comes from upstream and does NOT contain the changes for the S3 backend support. Please pin to a specific commmit from the `v4` branch if needed.