The fourth Developer Preview and 3rd beta of Android P has been released to those in the Android Beta Program today. The update is a few days later than expected. According to the timeline Google published when Android P was announced, Developer Preview 4 was slated for a late June release.
The update is just under 300 MB and is already rolling out to those who have devices registered in the Beta Program. I currently have my Google Pixel XL in the program and the OTA update has already been received and installed. As far as new things are concerned, you won’t find much. There are a tweaks here and there throughout the build but no major feature changes.