Apple has seeded beta 4 of Apple TV 5.4 to developers. Mum's the word on new features so far, but the beta is available through Apple's Developer site and through Apple TV devices already set up to run the developer betas.
Apple has been testing iOS 7, OS X Mavericks, Apple TV, and iTunes through the developer program since the company's World Wide Developer Conference in June. Only paid developer accounts have access to the betas.
If you notice something new in this beta, drop me a note and I'll add it to this story.