The first Developer Preview for Android Things OS was released at the end of 2016, as a lightweight version of Android intended for Internet of Things devices. Since then, it has been updated multiple times to support more hardware and fix bugs, and today Google has released Developer Preview 4.1.
As the minor version change might imply, this isn't a massive release. The first new feature is support for the Pico i.MX6UL revision B board, with support for earlier revisions (such as those made available to beta testers) being dropped. Secondly, DP4.1 includes a new smaller variant of Google Play Services specifically optimized for IoT devices.
Finally, there are some boot optimizations for i.MX7D-based hardware. Developers can find more information, as well as downloadable images for DP4.1, at the links below.