Why Apple watch 4 has not enabled ECG feature for the countries other than US?
The ECG function requires approval from the relevant regulators in the region, which takes time to acquire.


  In the US, Apple worked with the Food and Drug Administration (FDA) for a number of years to receive De Novo classification for the ECG app and the irregular heart rhythm notification, making the features available over the counter. The ECG app and irregular rhythm notification are now CE marked and cleared in the European Economic Area.


https://www.apple.com/uk/newsroom/2019/03/ecg-app-and-irregular-rhythm-notification-on-apple-watch-available-today-across-europe-and-hong-kong/

The functionality is available outside the US, namely…


  Austria, Belgium, Denmark, Finland, France, Germany, Greece, Guam, Hong Kong, Hungary, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Puerto Rico, Romania, Spain, Sweden, Switzerland, UK, US and US Virgin Islands