My mother hada persistant dry cough for about 4 years, what could be the reasons for that?

Cough 4 Years: A 4-year cough means chronic bronchitis, with many different associated possible causes. While a tumor is not likely, she needs to see her primary care provider -- now. She needs a chest x-ray, then if no mass is seen and there are no signs of heart failure, she should be treated for chronic bronchitis. If not better, suspect sinus disease, acid reflux, and / or allergies. She needs to see her doc.

See her doctor..: The chronic cough can be due to bronchospasm, post-nasal drip, gerd, or a combination of etiologies. For proper diagnosis and treatment, she needs to be evaluated by a doctor. If she is smoking, she needs to quit and her doctor can help with this. Good luck.