A leaked Pixel 4a (5G) prototype has shown that Google may have been experimenting with glass and aluminium versions of the device. As it stands, Google is expected to release the Pixel 4a (5G) with a plastic back, as it has with the Pixel 4a.
https://www.notebookcheck.net/Leaked-Google-Pixel-4a-5G-prototype-shows-how-glass-or-aluminium-models-could-have-looked.495658.0.html