It might seem like Google's Pixel 4 and 4 XL have been around for months, given the volume of leaks and early shipments, but today was actually meant to mark formal availability for the new phones. In addition to the hardware, today also marks availability for the Pixel 4 and 4 XL's software, as Google has just uploaded factory images and full OTA files for both phones.
Right now, you can download the latest/current software build (QD1A.190821.011.C4) with October's security patches, but two earlier versions have also been uploaded. All of the builds have the same date code, corresponding to October 21st, but the trailing modifiers are a bit different, and we aren't sure what software differences they might contain.
Either way, if you're curious, those earlier builds are also available to download and flash. Just scroll down to the new "coral" Pixel 4 XL and "flame" Pixel 4 sections at either of the pages listed below:
While the root-and-ROM enthusiast crowd might have a bit of a wait ahead of itself when it comes to TWRP custom recovery, the factory images required when things inevitably go wrong are now available.
Source: Factory Images, OTA Images