I have noticed a pretty good improvement with the heart rate sensor during workouts. With my series 2 and series 3, at least a few times per workout, I would have to adjust the band or take it off and clean off sweat so that it would work. I have had the S4 for a week now and have not had to do that one time. It seems to be more accurate as well. I’m just basing that on taking my own heart rate manually though so take that for what it’s worth.
Regardless, I have been very impressed with the AW4. I was not happy about paying more this time around, but I’ve gotten over it.