Hopefully others will chime in. I'm sorry that you ended up with sick birds.
Not to be harsh, but personally I would cull the 4 new birds, especially if there's an existing flock that seems healthy and has no history of respiratory illness.
Looks like the bird in the photo has respiratory disease - which one, hard to know without
necropsy. Mycoplasma is fairly common but so are a few others. Here's info about diseases
https://edis.ifas.ufl.edu/publication/PS044
Respiratory diseases are contagious and most make birds carriers for life. Disease is spread through dust, dander, feathers, shared equipment, bird to bird, etc. Disease like Mycoplasma can be passed along into hatching eggs.
If you feel you want to keep them, then of course, you can treat symptoms, keeping in mind that your existing flock may contract the illness either through you spreading it or through integration. For Mycoplasma, Tylan/Tylosin can be used to help with symptoms. Viral disease like Infectious Bronchitis, it would have to run it's course, but an antibiotic like Tylosin may help with secondary infection.
Is there a bad odor about the face or from the mucous?
What's the inside of the beak look like?
General care for the eye would be to flush with saline, remove bubbles/water/pus and apply Terramycin eye ointment to help with infection.