Welcome to Android Central! I don't know how likely Wear OS watches will incorporate ECG -- it might be something Fitbit or other fitness-specific bands might consider.
As a doctor, I've been regarding the whole adulation of the Series 4 with some bemusement. From what I can tell, the Series 4 requires you to press your finger to the digital crown for 30 seconds in order to get a reading (
https://www.tomsguide.com/us/apple-watch-series-4-ekg-sensor,news-28081.html). This would therefore only be useful for a patient who is already suspecting that they're currently in atrial fibrillation (presumably because they feel palpitations, or feel their radial pulse is irregular). For these kinds of patients, palpating an irregular pulse will likely be just as good as measuring an ECG (unless, perhaps, if the heart rate is going so fast that it's hard to tell if it's regular or irregular).
Where the watch could have been a game changer is if it could be monitoring the ECG
constantly in the background. A-fib can often be asymptomatic, so being able to alert someone about it when they're totally unaware of it would be extremely helpful. That's what Holter monitors or Ziopatches are used for, but those require multiple electrodes to be attached to the skin at all times. But since measuring an ECG requires contact points from at least two separate parts of the body, that's probably impossible for a standalone watch.
It'd also be helpful if the watch could detect ventricular arrhythmias, like ventricular tachycardia. However, in all of the materials and press I've seen about the Series 4, all I see mentioned is a-fib, which makes me think it isn't accurate enough to detect v-tach.