Maybe it is just a honeymoon phase...but I just did my first work with my series 4...I used my ProForm HIIT trainer and when I did that with my series 3 often when I would check my heart rate it would be searching for it but today with my series 4 it kept track of it each time I checked it. I will be interested to see if it can keep track of my heart rate better while I am doing my other workouts that involve different movements. Anyone else notice more accuracy?