I too have a series 4 and I mostly just use it to view the time - my biggest wish is for an always on display. I can often get 2 days out of it on a single charge. I asked a couple people in the Apple Store how their series 5 (always on) compared with that and they said the battery life in the series 5 is unquestionably less and there's no way you'd ever get 2 days out of a single charge. I also know a thing or two about lithium ion battery technology (having designed chargers for them) - fast charging, overcharging and letting them run flat does a number on their lifetime. If you want to wear the watch all day and use it for sleep tracking then the only solution is a fast charge while you're in the shower or some other such scenario. It's doable, but comes with a downside for the battery.