As I don't know how you have it setup. This will give a brief description of how it should look.
if health greater then 400 heart 1
if health less then 400 heart 2
if health greater then 200 heart 2
if health less then 200 heart 3
if health =0 then zero hearts
now you could get even more fancy
and set every 1/4 set animation to heart 1 (frame 1-4)
then that would call at every -50 since health is 600 and each heart has 200 points.