I was just reading again my Mavic Air user manual and came across the description of the flight telemetry.  For "H" it said "height from the Home Point".  Does this mean if the field I am flying over is 4 feet higher than my takeoff point marked as my home point, if I am flying fast over the field at a low altitude at what the display says 4 feet, I am actually ready to hit the ground?  I usually run with obstacle avoidance and all the safety settings but is there risk of me hitting the ground if I am in sports mode?  I always thought the height data was based on GPS or some other measurement which was actual height at the current location?  

No, it is indeed based on your take off location. Only way to avoid crash in your scenario would be flying with APAS activated, but this will not work in Sport mode.