I'm trying to decide between getting a Series 4 and a Series 3. Can you please tell me the difference?

The Series 3 is older and likely to be discontinued in September when the new model is released . The Series 4 features the ECG app in selected countries, fall detection and a slightly larger screen.

For more specific differences, see here: Apple Watch - Compare Models - Apple

Apart from a faster processor for better speed in the new model, and the ECG function, fall detection, and a few new faces which were added in Series 4, the practical differences are small but lots of them. You will find the differences in the comparison page on Apple's main website, Apple Watch - Compare Models - Apple

Apple Watch - Compare Models - Apple will be a good place to start.