Prepare release for opentelemetry-instrumentation-google-genai v0.4b0 (#3867)

Co-authored-by: otelbot <[email protected]>

Author: otelbot[bot]

instrumentation-genai/opentelemetry-instrumentation-google-genai/CHANGELOG.md

@@ -5,7 +5,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## Unreleased
+## Version 0.4b0 (2025-10-16)
 
 - Implement the new semantic convention changes made in https://github.com/open-telemetry/semantic-conventions/pull/2179.
 A single event (`gen_ai.client.inference.operation.details`) is used to capture Chat History. This is opt-in,

instrumentation-genai/opentelemetry-instrumentation-google-genai/src/opentelemetry/instrumentation/google_genai/version.py

@@ -17,4 +17,4 @@
 #   This version should stay below "1.0" until the fundamentals
 #   in "TODOS.md" have been addressed. Please revisit the TODOs
 #   listed there before bumping to a stable version.
-__version__ = "0.4b0.dev"
+__version__ = "0.4b0"