ECG enables the Series 4 to take a reading of your heart rate, via electrodes on the rear of the smartwatch, and on the new Digital Crown. The ECG app can then reveal abnormal patterns, and the 30 second ECG can be presented to your doctor via a downloadable PDF.
Essential reading: Best Apple Watch Series 4 apps to download first
The Watch feature was not available at launch, and crucially was only said to be limited to use in the US. That's down to the fact that Apple has FDA approval for the feature to be used to detect serious heart health issues such as atrial fibrillation.
Apple hasn't officially said the feature is good to go in other countries, but the sensors to enable it are of course already on board. According to the folks at 9to5Mac there is a workaround to get it running. All it apparently takes is to simply change your region to the US to activate it. Doing that though will impact on other Watch settings such as date/time formats and units of measurements. So that's definitely something to be mindful of if you're desperate to try it out.
As mentioned, the ECG feature on the Watch Series 4 has currently only been approved by the appropriate regulatory body in the US. It'll have to go through a similar process to get it the thumbs up to be used for serious health monitoring in other countries.
That could take a while though. In the UK for instance, it's believed it could take years to be approved with Apple's smartwatch likely to have to be used in a study as part of the approval process. That would help it get the sign off by the Medicines and Healthcare products Regulatory Agency (MHRA). That's the UK equivalent of the FDA.
So if you really want to see what ECG on the Watch Series 4 is all about, it looks like you can try it out now. Just be very aware that unless you live in the US, it's not been cleared to help detect serious health issues yet. Maybe it's one feature you wait for until Apple gives it the all clear.