it's not 4 regions, it's 8, 45 degree angles instead of 90.
as depicted below, the red zones indicate front/back, the blue zones indicate left and right, and the purple zones are spooky nebulous zones that include both the front/back and sides, and will be resolved as either one by chance.
1: the hit
VOXEL. the pixels you see on the screen are merely graphical representations, and are irrelevant in terms of resolving hits. the second picture i have included is how the game sees things internally. the purple cylinder is a floater, the yellow cylinder is a tank, and the green cylinders are street lights.
1.1: see
here2: the location of a wound is determined by which voxel is hit, again there is a random element to this determination, but higher shots will generally hit the head and lower ones will generally hit the legs, and shots from the left shouldn't damage limbs on the right.
3: see anwer to question 1.