Undoubtedly the most exciting feature of the new Apple Watch Series 4 is the ECG functionality. However, it isn't available yet and we are still waiting for Apple to enable it via a future update. And according to inside sources, watchOS 5.1.2 is going to be the one to bring it.
Apple Watch Series 4 to get ECG suport with the public release of watchOS 5.1.2
watchOS 5.1.2 will add an Apple-designed ECG app which is capable of indicating heart rhythm anomalies.