Respiratory infections are pretty common in chickens but they can also be susceptible to dust and ammonia from droppings, so the first thing to do would be to assess their environment and your management system and see if the latter may be issues and what you can do to improve it.
Infections and diseases, particularly viral ones, have to run their course. Antibiotics will help with a bacterial infection, but plenty of times, birds will recover without antibiotics and personally I believe many people are too keen to use them at the slightest sign of respiratory problems when in my experience, birds are quite able to fight off many infections. Vet RX can be helpful in alleviating the symptoms and opening up their airways if they are congested or even Vicks Vapour Rub, but generally I ensure they have good nutrition, perhaps give them a poultry vitamin supplement to support the immune system and monitor the bird, but accept it has to run it's course and only consider medicinal intervention if the bird is clearly deteriorating and distressed.
If you think of yourself with a common cold.... it's a viral infection.... you cough and sneeze and perhaps wheeze and have a snotty nose, but there is generally no necessity for antibiotics and after a couple of weeks or maybe even a month, it is gone..... chickens are no different in that they will pick up viruses and infections just like we do.
You have the option to isolate the bird to try to prevent the other 3 from getting it but then you will have the problem of reintroduction at a later date and isolated hens usually get depressed and that compromises the immune system. I leave the flock together and let these things run their course, but everyone has their own opinion and I am sure others will express theirs, so that you can then make an informed decision.
Best wishes
Barbara
