Bump version to 0.4.0 for new release.

PiperOrigin-RevId: 323448628

Author: alanchiao

tensorflow_model_optimization/python/core/version.py

@@ -16,7 +16,7 @@
 
 # We follow Semantic Versioning (https://semver.org/)
 _MAJOR_VERSION = '0'
-_MINOR_VERSION = '3'
+_MINOR_VERSION = '4'
 _PATCH_VERSION = '0'
 
 # When building releases, we can update this value on the release branch to