First of all, instead of glass, we are likely to see plastic Pixels once again in early 2020. It is highly likely that if the Pixel 4a and 4a XL come in the same color variations as the Pixel 4-series, they will also retain the material finishes at the rear. The Clearly White model, as well as the limited edition Oh So Orange, have the matte finish with a frosted look and a soft grip. The Just Black model, on the other hand, would probably be glossier. These properties are completely achievable with plastic, so the new Pixels would mostly feel like the  more premium devices at a lower weight. 

Source: Youtube