Next version of iOS with overhauled notifications, lock screen, Messages, Apple Music, and much more.
iOS 8.4 in Development With Possible Support for New Music Service
Given the release timing after iOS 8.2 and iOS 8.3, it's possible that iOS 8.4 will be the update that introduces Apple's new streaming music service. Recent rumors have suggested that Apple's existing Beats music service will be rebranded, revamped with a lower price tag, and integrated into iOS and OS X. A timeline is unclear, but Apple could be aiming for a June launch, sometime around its annual Worldwide Developers Conference.
iOS 8.4 began appearing in MacRumors site logs towards the end of January, with usage spiking up at the beginning of February. The number of visits from devices running iOS 8.4 from both Apple IPs and non-Apple IPs remains relatively low, however, suggesting that development on iOS 8.4 is in the very early stages.
iOS 8.2 has been in developer testing since November, and iOS 8.3, with wireless CarPlay support, a new emoji picker, and Apple Pay for China was just seeded to developers this morning. Along with its iOS 8 projects, Apple is also working on iOS 9, an update that may heavily focus on stability and optimization.