We already know that the Pixel 4 will have multiple rear cameras because Google confirmed the device last month. Google didn't tell us anything about what those cameras can do, but a new leak claims to have some info on the matter.
Code in the Google Camera app suggests that the Pixel 4 will have a telephoto camera. A list of sensor IDs has been found in Google Camera build 6.3 by XDA-Developers that includes "FRONT_REGULAR", "FRONT_WIDE", "REAR_REGULAR", and "REAR_TELEPHOTO".
That's not the only reference to a telephoto lens in the upcoming Google Camera app. In a section for "Sabre", Google's codename for its Super Res Zoom feature, there's a line that reads "SABRE_UNZOOMED_TELEPHOTO".
None of the Pixel phones release to date have a telephoto rear camera, so these telephoto references in the Google Camera app could definitely suggest that the Pixel 4 will have a rear telephoto camera in addition to its standard camera. These references are new in the Google Camera 6.3 app, suggesting that Google is preparing for the release of its new Pixel phones this fall.
While Google's Pixel phones are some of the best smartphone cameras around using just one rear camera sensor, it'd be great to see Google add a telephoto camera to the Pixel 4. It's a practical feature that helps folks get closer to their subject without physically moving or using digital zoom, and it's something we're seeing on a lot of other flagship smartphones lately.
UPDATE: After doing a bit of digging, 9to5Google found code that suggests that the Pixel 4's telephoto camera could be a 16MP sensor.