https://www.cnet.com/news/apple-watch-series-4-vs-series-3-which-one-should-you-buy/

The Series 4 has a slimmer body, but its screen is actually 35 percent larger than the 38-millimeter version of the Series 3 and 32 percent larger than the larger 42mm Series 3. The screen on the Apple Watch 4 has grown to 40mm and extends almost edge to edge with rounded corners, contrasting with the thicker bezels and square corners of the Series 3. This means you'll have more finger space to type in your passcode on the 4. Plus you have more watch faces to choose from.

The Series 4 takes advantage of the extra screen real estate by adding more watch complications on some of the watch faces allowing you to see more information at a glance. It also seems brighter and sharper than the previous model and was easier to read in direct sunlight.