iOS 14.4 beta 1 is now available to registered developers (via MacRumors). If you're not a dev, you can enroll in the Apple Beta Software Program for public testers to try new updates out before they're released to the public. iOS 14.4 beta 1 isn't rolling out to public testers quite yet, but it should begin appearing in the coming days.
Because this is the first beta release of iOS 14.4, there's no word yet on what new features or changes might be included. Because this is the fourth major version of iOS 14, it's likely that it'll be more focused on under-the-hood performance improvements and bug fixes, but Apple could sneak a new thing or two into the update.
Stay tuned for more info on what's packed into iOS 14.4 as that info becomes available.