So with my iphone I used the Zephyr smart Bluetooth heart rate monitor. I get my S4 and it dies. They send me a brand new one and it won't even find it when. I scan for devices. It is compatible with my os 4.3. I'm aggravated because I use this so much. I ordered a different one off amazon that a friend recommended but still I was so bummed. I've tried everything to get my phone to read it and I come up empty handed. ~ Beth ~ "Be nice to those you meet on the way up because you will meet them on the way down."