In my experience, it is not something set in stone, and most humans do have a bit of all four in them. The element that they represent simply represents the correspondence of their soul, or their godform, or something else.
And these can be more than one element, and they can fluctuate and change. Sometimes, though not always, one may turn more towards one. Sometimes that can stay forever, sometimes that can change, sometimes it's more than one. Sometimes it's tied to astrological factors, sometimes they're tetrapolar.
Depends on the individual. Different strokes for different people. Everyone has different experiences, and beliefs