Can a sprite atlas be 4 bit?

No matter what sprite I atlas using the new sprite atlases in 2017, it seems to be the case that the atlas always has an alpha channel. Even atlasing a single 512x512, 4 bit sprite (no alpha) creates a 512, 8 bit atlas.

The Problem: atlases don't seem to have an option to disable their alpha channel, even if they are composed entirely of 4bit sprites.

The Question: Can atlases be forced down to 4 bit, instead of always being 8? And does this truly make a difference if it's being built to Android & iOS?