Update opentelemetry-opamp-client version to v0.4b0 (#4812)

* Update opentelemetry-opamp-client version to v0.4b0

* Apply suggestion from @xrmx

---------

Co-authored-by: otelbot <197425009+otelbot@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>

Author: otelbot[bot]

opamp/opentelemetry-opamp-client/.changelog/4536.added

@@ -15,6 +15,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- changelog start -->
 
+## Version 0.3b0 (2026-07-16)
+
+### Added
+
+- `opentelemetry-opamp-client`: support non-JSON effective config bodies
+  ([#4536](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/4536))
+
 ## Version 0.2b0 (2026-04-01)
 
 - Breaking change: callback class `Callbacks` renamed to `OpAMPCallbacks`

opamp/opentelemetry-opamp-client/CHANGELOG.md

@@ -1,4 +1,4 @@
 # Copyright The OpenTelemetry Authors
 # SPDX-License-Identifier: Apache-2.0
 
-__version__ = "0.3b0.dev"
+__version__ = "0.4b0.dev"