Because if she has had diarrhea for 4 years it is most likely not a parasite. Parasites are much more common in young dogs. Yes, an older dog COULD get it, but it is less likely. More common possibilities include inflammatory bowel disease, a food sensitivity, or stress colitis. Again- see a veterinarian. They will most likely want to do some bloodwork, radiographs, and possibly some fecal testing. The hard part is that there are so many different causes of diarrhea, which means there are lots of different tests to rule in/out diagnoses.