I'm not certain you could get 3-days with the watch off but that said why buy the watch if you turn everything off? There are cheaper options that will get close to this watch.
Me, I could get 1-1/2 days, without serious workouts and that was it for the watch. Too many times I wound up putting it on the charger in the middle of the day. Not sure about the difference between the 4 and 5 because the 4 could do 2 days and I had AOD off.
I'll probably get a 6 though. You can always eBay them or sell it direct to a 3rd party and get half the purchase back if I hate the battery. And, right now I kind of wish I had my 5 back. It looks like I'll be going lighter on the workouts for a bit and the AW is better for that than my Garmin.
I turned AOD off and can get 2 days out of mine with about 8% left. I found AOD is worth about 30% of the battery in a 16 hour period.