Release notes: 4.1.0-beta.3 is out, bump to 4.1.0-beta.4

Author: michaelklishin

release-notes/4.1.0.md

@@ -1,6 +1,6 @@
-## RabbitMQ 4.1.0-beta.2
+## RabbitMQ 4.1.0-beta.4
 
-RabbitMQ 4.1.0-beta.2 is a preview release (in development) of a new feature release.
+RabbitMQ 4.1.0-beta.4 is a preview release (in development) of a new feature release.
 
 See Compatibility Notes below to learn about **breaking or potentially breaking changes** in this release.