tvOS betas are more difficult to install than beta updates for iOS and OS X. Installing the tvOS beta requires the Apple TV to be connected to a computer with a USB-C to USB-A cable, with the software downloaded and installed via iTunes or Apple Configurator. Those who have already installed the first three tvOS 9.2.1 betas will be able to update to the fourth beta over the air.
As a minor 9.x.x update, tvOS 9.2.1 focuses on bug fixes and performance improvements that are not readily apparent to Apple TV users. There have been no outward-facing changes in the first three tvOS 9.2.1 betas, but any new features discovered in the fourth beta will be listed below.