While most developers are probably already giving iOS 9 a test drive, Apple is still working on the upcoming iOS 8.4 update and just released a new version. iOS 8.4 build 12H4125a is now available on Apple’s developer center and as an OTA update for testers on the previous version.

Today’s release marks the fourth beta version of iOS 8.4 after first being seeded out in April. The update includes a redesigned Music app, audiobook playback through iBooks and CarPlay, and iBooks Author textbook support on iPhone.

Apple Music, the company’s new subscription music service, launches later this month on June 30th. iOS 8.4’s public release will likely be timed ahead of the new service’s debut.

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