Apple has released the fourth beta version of iOS 9.2 to developers. Developers can grab this new build from the iOS Developer Resource Center, or as an over-the-air update on an iPhone or iPad running the previous beta.
Apple has also released the fourth public beta version to members of its Beta Software Program. It is available as an over-the-air download through Settings > General > Software Update.
Not a lot is known about the exact changes in iOS 9.2. You can probably bet on some bug fixes and performance improvements to the overall iOS experience.