A quick checkup on the all-ceramic-and-sapphire-crystal back details:

 Watch, check.

Series 4, check.

A laundry list of features, including water resistance to 50 meters.

Optical sensor, right in the middle this time.

Big ring-shaped electrical sensor—that's new.

Inside one of the wrist band slots, we find a new model number, A2008—and in the other, the serial number, plus a small air vent hole for pressure equalization.