A competent vet is the best thing, it is sounding like your little one may have a respiratory infection, and that requires oral or injectable antibiotics. Please if you can get someone familiar with waterfowl to see your duckling. If you cannot, I would get antibiotics. I am not an expert on which kinds of antibiotics and the dosages, but here is one resource.I also have four week old Pekins. One started showing slight signs of a niacin deficiency one day so I switched them to a feed with more niacin and started supplementing with vitamins/niacin in their water. It has been over a week since I started supplementing them and the one has gotten worse. It cannot stand, does not move, eat or drink, and seems to be weezing. Help please!
https://sites.google.com/a/poultrypedia.com/poultrypedia/medicine-chart