After releasing iOS 10.0.3 to the public today, Apple released the fourth beta of iOS 10.1, bringing the build number to 14B72/14B72b. If you're coming from beta 3, it's a small update, which should come in at around 30MB.
Today's build comes exactly one week after the last one, which came alongside a third tvOS 10.0.1 beta. Since there was no tvOS build today, it would seem that all of Apple's operating systems are on different paces for beta releases.
The big new feature in iOS 10.1 will be Portrait Mode for the iPhone 7 Plus. This utilizes the phone's dual camera to create a blur effect in the background of an image.
Another feature that has shown up in an earlier build is a cleaner experience when scrolling between different extensions in Messages. You also might have found that the new animated effects in the app don't work if you have reduced motion settings turned on, but now you'll have the option to turn on those animations.
The release notes don't list anything new for beta 4, and since it's such a small update, it's probably not much. If you're already on the beta, you can grab today's build by heading over to Settings -> General -> Software Update.