The iOS 8.4 beta is out now and available for download via Apple's developer portal.
The biggest new feature appears to be a revamped Music app that includes a streamlined version of iTunes Radio. It comes amid rumors that Apple is set to debut a new music streaming service sometime around WWDC 2015 this June.
The developer release of the iOS 8.4 beta comes just under a week after Apple released iOS 8.3 to the public, so the company appears to be intent on keeping up momentum through the spring.
While today's beta release is developer-only, iOS 8.3 went through a period of invite-only public beta testing, so it's possible that iOS 8.4 may do the same.