Which statement describes HTTP idempotency correctly?

Which statement describes HTTP idempotency correctly?

1. Repeated identical requests have the same intended server effect as one request, although responses may differ.
2. Every idempotent request is read-only and has no server-side effects.
3. An operation is idempotent only when every retry returns the same status and body.
4. POST is automatically idempotent whenever it returns 201.