My kids and I are at Disneyland today and my 12 year old is wearing my series 3...it is set up for his height and weight. I am wearing my series 4. We have been walking together all day. My watch says I have burned 250 calories more. Is that because my heart beats more quickly than his? I do feel the series 4 is more generous in getting me to my move goal but this seems to be a huge difference!