Buried in Apple’s official Apple Watch Series 4 site is this page of videos, all worth a look.
One interesting bit, buried about 47 seconds into that last video, in a faint grey font. Take a look:
There’s a reference to the 5s, which might suggest that this is sourced from older video. And I don’t see the “not a medical device” warning in the other videos, even those that talk about heart rate and the ECG sensor.
That nit aside, these are fantastically produced videos, just gorgeous. Give ’em a look.