The four dots represent the heart rate zones.
The heart rate zones cover the range between your resting heart rate and your maximum heart rate and can help you identify the intensity of your exercise.
Your heart rate zone is displayed in your heart rate screen with a heart icon and four dots.  

Heart rate zone 1 - Light
Heart rate zone 2 - Moderate
Heart rate zone 3 - High
Heart rate zone 4 - Anaerobic