Android 9 Pie introduced the first form of gesture navigation, changing the three-button navigation bar into a pill-shaped button with a separate back key. Android 10 came with a new full-screen gesture navigation mode, and if you get a Pixel 4, it's the only option — besides the classic three-button nav, that is.
Left: Pixel 3 updated to Android 10; Right: Pixel 4
Pixel phones updated from Pie to Android 10 get two choices for gesture navigation: the previous two-button system, or Android 10's new full-screen gestures. However, only the latter is available on the Pixel 4. On a similar note, none of OnePlus' phones updated to Android 10 offer the two-button navigation either.
While about 11% of our readership preferred Pie's gestures over all other options in a recent poll (which was conducted before Android 10's implementation was finalized), it makes sense why the option is missing on the newest Pixel. Google likely wants to put all previous attempts at gesture navigation behind it, especially since the company believes the latest version is much better.