I know this feeling about trying to help your Mother without any results. If you are trying to talk her into seeing a physician, certainly try to go straight to a Pulmonologist, which specializes in lung disease and conditions. There are many lung conditions that can attribute to a persistent cough but as Susan stated, 4 years is way to long.
It is very important that she get treatment or at least a diagnosis since there are so many things that you can be exposed to that will cause chronic lung conditions. I happen to work in a field that specializes with work related lung conditions due to exposures. Since she works at a Dollar Store and may or may not be exposed to chemicals, there is always a chance that something in the home may be causing the condition as well, such as Black Mold.
Please keep us updated and continually talk to her and get your other family members involved to try to get her medical attention.