All of the ones in my photos above are 4 feet.
[:-^]
All buildings and loading areas (team tracks) are not necessarily on absolute level ground, so a better rule of thumb to use is to make the dock as high as it has to be to get from the ground to the bottom of the door on a box car sitting on the tracks (on one side) and as high as it has to be to get from the ground to the warehouse floor on the other (simplified but works in most cases you will find.)
Johnboy out…