It's showing up a little later than we expected, but the fourth Android P Developer Preview is rolling out today. It's available on the first and second generation Pixel phones as both a system image and an OTA file. However, the Android beta program updates aren't going out quite yet.
You can install the factory images on a Pixel with an unlocked bootloader, and it doesn't matter if you've been on the developer preview previously. The OTA files, meanwhile, will upgrade your DP3-based Pixel to the latest build. The OTA doesn't require an unlocked bootloader. If you're in the Android beta program, you should get an upgrade notification in the not too distant future if you don't mind waiting.
This one is slightly late.
Google's release notes for DP4 (which is also Beta 3) notes that there could be battery and stability issues, but there aren't as many specific warnings as in past previews. As always, installing beta software is done at your own risk.