I think that the vet that examined your birds would have identified any parasite problem that you might have. Production birds do seem to have a shorter life span, but really a 3-4 year old chicken is a pretty old chicken, some people have them live much longer, but often times they don't.
If these were your original chickens, and you started with them 4 years ago, I think that there is a good chance that they just died of old age, and the others might do so too soon. I don't think you need medicine, I think you need new chicks!
Mrs K