Four years of diarrhea
, that's a long time. Medication can often cause that, review what you're taking with your doc, and perhaps change kinds. Changing eating habits can help diarrhea, more fiber, and enough food in general.
As to the pain you feel in your lower stomach and back, that's about the location of the kidneys, so could be you have a problem there, altho I imagine it may connected to your diarrhea.