It’s bigger, it’s better-looking and it could save your life
The larger screen (30% larger no less), especially as the Watch itself has barely grown at all. The Series 4 comes in 40mm and 44mm versions (measured vertically), compared to 38mm and 42mm for past Watches.
There’s no native sleep tracking, and even if there was the battery life is still too short to allow you to wear the Watch at night. The battery will last a day comfortably, but not two, so you have to plug it in every night.