The December 2019 update finally started rolling out for the Pixel 4 and 4 XL yesterday. If you're among those impatiently tapping the "check for updates" button to no avail, you can now download the images manually and install them yourself from a computer, as OTA and factory image files have been posted by Google.
While you'll need an unlocked bootloader to flash the full factory images, even those of you with stock, locked devices can sideload the OTA manually with the help of a computer. It's not difficult, just a little technical and tedious. Be sure to follow Google's instructions at the top of the OTA Downloads page if you aren't familiar with the process.
You will also need to be careful about which images you download. Google has divided things up a bit by carrier this time around, with a single image for most of us, paired with two others that cover AT&T and some other carriers. If you install the wrong one, you could run into issues. So if you're worried, it may be best to wait for the OTA to roll out via the traditional means.