If it's only been 3-4 weeks, then your in that 1 month QT time as I like to call it. It's why QT'ing new stock for a month is recommended since some illnesses can take that long to show. With Neons what your seeing is typical of what I know about them. They'll do fine for weeks and then loss of color, rapid breathing, then they drop like flies.
Hopefully that will not be in your case.