It appears that Google is rolling out the option for Pixel 4 owners to use the old style of three-button navigation while still getting access to the new Assistant.
The update seems like a server-side rollout, according to Android Police, but the publication mentions that having your Google app updated to version 10.92.10.29 might help.
If your phone works with the new Assistant and three-button navigation, all you need to do is hold on the home button to trigger it. Alternatively, you can use the “Ok Google” hot word or squeeze the phone.
You can learn more about how the new Assistant differs from the older one by reading our prior reporting.
Source: Android Police