Sadly, I agree with @speckledhen . It would be nice if your girl was JUST going broody though!
I would not take her to the vet for reasons already mentioned. My vet charges $45 to see a bird and $25 for a fecal float test. But with time and experience I also am becoming my own vet. Bought myself a high quality microscope to do my own float tests. And read how to make the float liquid. With 49 chickens, 3 dogs, and 2 goats... diagnostics can get pricey!
Speaking of diagnostics.... Have you checked for mites, treated for worms or anything like that?
They didn't carry the poly vi sol at my wally and what they had to offer didn't include the B vitamins which I think are key. So I just got some bird vitamins and mix it with a homemade electrolyte solution. My version I guess of Sav-a-Chick. It really seem s to help with my shipped birds! It will not cure anything, but it MIGHT help provide enough energy for the hen to fight off or recover from whatever she is going through.
I know we have all bought hatchery stock. And I don't think any bashing on them is intended. The things @speckledhen says are simple truths. For people who don't have long term breeding goals they are fine. All my egg layers are hatchery stock of different breeds. My long term birds all came from breeders..... and there are different qualities of those as well.
4 years seems like a good lifespan. It can be effected by many factors including feed, breed, housing conditions, predation, and even the weather.
Listen hear now.... IF that girl IS broody.... I wanna see some pics please?!