Docs

docs/migrate_v3_v4.md

@@ -44,4 +44,5 @@ Any `Either`-based response description can be converted to a failing one using
 * the parametrization of `ResponseException` is simplified, `DeserializationException` does not have a type parameter, always requiring an `Exception` as the cause instead
 * when a `ResponseException` is thrown in the response handling specification, this will be logged as a successful response (as the response was received correctly), and counted as a success in the metrics as well
 * `HttpError` is renamed to `UnexpectedStatusCode`, and along with `DeserializationException`, both types are nested within `ResponseException`
-* the `opentelemetry-metrics-backend` module is renamed to `opentelemetry-backend`
+* the `opentelemetry-metrics-backend` module is renamed to `opentelemetry-backend`
+* `ListenerBackend` has been refactored, with improvements in naming, and additional callback methods