Apple today seeded the first beta of iOS 8.4 to registered developers for testing purposes, just five days after releasing iOS 8.3 to the public. The beta is available for download from the iOS Developer Center.iOS 8.3 introduced several new features like diversified emoji, an updated emoji picker, new Siri abilities, and Wireless CarPlay support. It is not clear yet what updates iOS 8.4 will bring, but it has been rumored to be the update that will include Apple's new music service.
Apple Seeds First iOS 8.4 Beta to Developers